Support WebDriver takeScreenshot command.

[jgraham]

This adds support for compositing to a PNG without actually quiting
the browser.

Cargo bits need to be updated after the upstream changes to rust-png land.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4837

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[bors-servo]

☔ The latest upstream changes (presumably #5895) made this pull request unmergeable. Please resolve the merge conflicts.

[jdm]

Reviewed files:

• components/compositing/compositor.rs @ r1
• components/compositing/compositor_task.rs @ r1
• components/compositing/constellation.rs @ r1
• components/compositing/headless.rs @ r1
• components/msg/Cargo.toml @ r1
• components/msg/constellation_msg.rs @ r1
• components/msg/lib.rs @ r1
• components/script/script_task.rs @ r1
• components/servo/Cargo.lock @ r1
• components/webdriver_server/Cargo.toml @ r1
• components/webdriver_server/lib.rs @ r1

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^{components/compositing/compositor.rs, line 191 [r1] (raw file):}
This option deserves documentation, so we should probably document the others and the overall type as well.

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^{components/compositing/compositor.rs, line 1148 [r1] (raw file):}
Would it make more sense to take an Option<Sender<png::Image>> argument instead of having all of these return None branches?

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^{components/compositing/compositor.rs, line 1219 [r1] (raw file):}
This can just be a free function, and I think that will be clearer too.

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^{components/compositing/compositor_task.rs, line 221 [r1] (raw file):}
This is very zen.

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^{components/msg/constellation_msg.rs, line 238 [r1] (raw file):}
/// for docs (and the one above

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^{components/webdriver_server/lib.rs, line 11 [r1] (raw file):}
I believe this is gone in the Rust upgrade that is coming down the pipes. I'd like to find a different solution.

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^{components/webdriver_server/lib.rs, line 47 [r1] (raw file):}
std:🧵:sleep_ms?

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^{components/webdriver_server/lib.rs, line 161 [r1] (raw file):}
for _ in &[0..iterations] {

---

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Comments from the review on Reviewable.io

[jdm]

-S-awaiting-review +S-needs-code-changes +S-needs-rebase

---

Comments from the review on Reviewable.io

[jgraham]

^{components/compositing/compositor.rs, line 1148 [r1] (raw file):}
I sort of felt like this was a better separation of concerns because it doesn't bake sending the PNG over a channel into this function. But I can do it the other way if you prefer.

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^{components/webdriver_server/lib.rs, line 11 [r1] (raw file):}
Yeah it is; after the rustup I can use std:🧵:sleep_ms as you point out below. But afaict from the docs, that doesn't exist in the current version of rustc. Although, perhaps I should verify that rather than trusting them.

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Comments from the review on Reviewable.io

[jdm]

Reviewable is confusing me but I have marked those two as addressed, based on your feedback.

---

Comments from the review on Reviewable.io

[jdm]

-S-needs-rebase -S-awaiting-review +S-needs-code-changes

---

Reviewed files:

• components/compositing/compositor.rs @ r2
• components/compositing/compositor_task.rs @ r2
• components/compositing/constellation.rs @ r2
• components/compositing/headless.rs @ r2
• components/msg/constellation_msg.rs @ r2
• components/script/script_task.rs @ r2
• components/servo/Cargo.lock @ r2
• components/webdriver_server/lib.rs @ r2

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^{components/webdriver_server/lib.rs, line 166 [r2] (raw file):}
interval?

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Comments from the review on Reviewable.io

[bors-servo]

☔ The latest upstream changes (presumably #5753) made this pull request unmergeable. Please resolve the merge conflicts.

[jdm]

-S-awaiting-review +S-needs-squash

---

Reviewed files:

• components/compositing/compositor.rs @ r3
• components/compositing/compositor_task.rs @ r3
• components/compositing/constellation.rs @ r3
• components/msg/constellation_msg.rs @ r3
• components/script/script_task.rs @ r3
• components/servo/Cargo.lock @ r3
• components/webdriver_server/Cargo.toml @ r3
• components/webdriver_server/lib.rs @ r3

---

Comments from the review on Reviewable.io

[bors-servo]

☔ The latest upstream changes (presumably #5361) made this pull request unmergeable. Please resolve the merge conflicts.

[jdm]

@bors-servo: r+
-S-awaiting-review -S-needs-rebase -S-needs-squash

---

Reviewed files:

• components/compositing/compositor.rs @ r4
• components/compositing/compositor_task.rs @ r4
• components/compositing/constellation.rs @ r4
• components/compositing/headless.rs @ r4
• components/msg/constellation_msg.rs @ r4
• components/msg/lib.rs @ r4
• components/script/script_task.rs @ r4
• components/servo/Cargo.lock @ r4
• components/webdriver_server/lib.rs @ r4

---

Comments from the review on Reviewable.io

[bors-servo]

📌 Commit 570aa4d has been approved by jdm

[bors-servo]

⌛ Testing commit 570aa4d with merge 3989bb4...

[bors-servo]

💔 Test failed - mac1

[bors-servo]

☔ The latest upstream changes (presumably #5968) made this pull request unmergeable. Please resolve the merge conflicts.

[Ms2ger]

@bors-servo: r=jdm

[bors-servo]

📌 Commit 967edc3 has been approved by jdm

[bors-servo]

⌛ Testing commit 967edc3 with merge 74ead3f...

[bors-servo]

💔 Test failed - gonk

[Ms2ger]

@bors-servo: r=jdm

[bors-servo]

📌 Commit 5311d9a has been approved by jdm

[bors-servo]

⌛ Testing commit 5311d9a with merge 63ba1cb...

[bors-servo]

☀️ Test successful - android, gonk, linux1, linux2, mac1, mac2