Add WebDriver support for getting elements by selector.

[jgraham]

Also adds example support for getting the name and text properties of the elements.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4927

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[jdm]

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

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Reviewed files:

• components/script/script_task.rs @ r1
• components/script/webdriver_handlers.rs @ r1
• components/webdriver_server/lib.rs @ r1
• components/webdriver_traits/lib.rs @ r1

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^{components/script/webdriver_handlers.rs, line 28 [r1] (raw file):}
nit: no need for : JSRef<Node>.

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^{components/script/webdriver_handlers.rs, line 42 [r1] (raw file):}
Let's make get_unique_id always initialize the unique id if it isn't set yet, instead.

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^{components/script/webdriver_handlers.rs, line 105 [r1] (raw file):}
unwrap_or("".to_owned())

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^{components/script/webdriver_handlers.rs, line 115 [r1] (raw file):}
nit: no need for : JSRef<Element>

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^{components/webdriver_server/lib.rs, line 162 [r1] (raw file):}
Let's move the command into a separate binding; this is a pretty long line.

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^{components/webdriver_server/lib.rs, line 183 [r1] (raw file):}
Same here.

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^{components/webdriver_server/lib.rs, line 186 [r1] (raw file):}
If you use into_iter() you don't need x.clone().

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^{components/webdriver_server/lib.rs, line 201 [r1] (raw file):}
Long line, separate binding.

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^{components/webdriver_server/lib.rs, line 215 [r1] (raw file):}
Long line; separate binding.

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Comments from the review on Reviewable.io

[jgraham]

^{components/script/webdriver_handlers.rs, line 105 [r1] (raw file):}
It's not supposed to be possible to select non-Elements through the API, but I guess this can't hurt.

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Comments from the review on Reviewable.io

[jdm]

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

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Reviewed files:

• components/script/dom/node.rs @ r2
• components/script/webdriver_handlers.rs @ r2
• components/webdriver_server/lib.rs @ r2

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Comments from the review on Reviewable.io

[jdm]

@bors-servo: r+

[bors-servo]

📌 Commit 9e44206 has been approved by jdm

[bors-servo]

⌛ Testing commit 9e44206 with merge 8df8249...

[bors-servo]

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