Implement cancellable runnables, and make image load events cancellab…

[jdm]

…le. Resolves #7731.

[jdm]

r? @nox

[nox]

Seems good; just one nit and question. If you could rewrite the commit message to a short title and a body it would be perfect.

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

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Reviewed 4 of 4 files at r1.
Review status: all files reviewed at latest revision, 2 unresolved discussions, all commit checks successful.

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^{components/script/dom/window.rs, line 69 [r1] (raw file):}
Nit: that space.

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^{components/script/script_task.rs, line 170 [r1] (raw file):}
Couldn't we avoid the nested boxes, somehow?

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Comments from the review on Reviewable.io

[jdm]

Review status: 1 of 4 files reviewed at latest revision, 2 unresolved discussions, all commit checks successful.

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^{components/script/script_task.rs, line 170 [r1] (raw file):}
I can't figure out how, since Runnable::handler requires self: Box<Self> and I can't call it without inner being boxed.

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Comments from the review on Reviewable.io

[bors-servo]

☔ The latest upstream changes (presumably #8056) made this pull request unmergeable. Please resolve the merge conflicts.

[jdm]

Nit fixed. Let's kill off an annoying intermittent test failure!

[Ms2ger]

@bors-servo r=nox

[bors-servo]

📌 Commit 989e800 has been approved by nox

[bors-servo]

⌛ Testing commit 989e800 with merge 4113302...

[bors-servo]

☀️ Test successful - android, gonk, linux-dev, linux-rel, mac-dev-ref-unit, mac-rel-css, mac-rel-wpt