Fix several hangs / panics during pipeline cleanup of in progress loads.

[glennw]

This fixes a hang found while testing the jQuery test suite.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/5047

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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.

[glennw]

r? @jdm or @larsbergstrom

[jdm]

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

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Review status: all files reviewed, 2 unresolved discussions, all commit checks successful.
Reviewed files:

• components/compositing/compositor_task.rs @ r1
• components/gfx/paint_task.rs @ r1
• components/layout/layout_task.rs @ r1
• components/msg/constellation_msg.rs @ r1
• components/script/dom/window.rs @ r1
• components/script/script_task.rs @ r1
• components/util/task.rs @ r1

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^{components/script/script_task.rs, line 1089 [r1] (raw file):}
This comment's position is misleading now.

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^{components/script/script_task.rs, line 1097 [r1] (raw file):}
This is not necessarily the root.

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Comments from the review on Reviewable.io

[glennw]

Review status: 6 of 7 files reviewed, 2 unresolved discussions, all commit checks successful.

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^{components/script/script_task.rs, line 1089 [r1] (raw file):}
Done.

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^{components/script/script_task.rs, line 1097 [r1] (raw file):}
Done.

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Comments from the review on Reviewable.io

[jdm]

@bors-servo: r+
-S-awaiting-review +S-awaiting-merge

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Review status: all files reviewed, all discussions resolved, all commit checks successful.
Reviewed files:

• components/script/script_task.rs @ r4

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Comments from the review on Reviewable.io

[bors-servo]

📌 Commit 41c243e has been approved by jdm

[bors-servo]

⌛ Testing commit 41c243e with merge c51e9f0...

[bors-servo]

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