Don't crash when Firefox doesn't send autoMargins.

[notriddle]

Firefox Developer Tools doesn't always send autoMargins, as seen in this message:

{"type":"getLayout","node":"node14","to":"pageStyle9"}

So don't use unwrap.

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @Ms2ger (or someone else) soon.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/5563

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[jdm]

Thanks @notriddle! Be sure to check the output of ./mach test-tidy after making these changes, too!
-S-awaiting-review +S-needs-code-changes

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Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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^{components/devtools/actors/inspector.rs, line 462 [r1] (raw file):}

let auto_margins = msg.get(&"autoMargins".to_string()).and_then(&Json::as_boolean)).unwrap_or(false);

We don't want to include any margin information if it's not requested.

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Comments from the review on Reviewable.io

[jdm]

Looks great! Could you squash these three commits together, please?
-S-awaiting-review +S-needs-squash

---

Reviewed 1 of 1 files at r3.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

---

Comments from the review on Reviewable.io

[notriddle]

It's squashed.

[jdm]

@bors-servo: r+
Thanks!

[bors-servo]

📌 Commit 602c8bb has been approved by jdm

[bors-servo]

⌛ Testing commit 602c8bb with merge b9ac4bf...

[bors-servo]

💔 Test failed - mac3

[jdm]

@bors-servo: retry

• #6644

[bors-servo]

⌛ Testing commit 602c8bb with merge 10f3a89...

[bors-servo]

💔 Test failed - mac3

[jdm]

@bors-servo: #6644

[jdm]

@bors-servo: retry

[bors-servo]

⚡ Previous build results for android, gonk, linux1, linux2, linux3, mac1, mac2 are reusable. Rebuilding only mac3...

[bors-servo]

☀️ Test successful - android, gonk, linux1, linux2, linux3, mac1, mac2, mac3