Introduce DOMClass::heap_size_of

[nox]

It holds a function pointer to the HeapSizeOf::heap_size_of_children()
implementation corresponding to that IDL interface.

This removes the need for a clumsly TypeId-based match expression in the
former heap_size_of_eventtarget() function.

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[Ms2ger]

-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, 4 unresolved discussions, all commit checks successful.

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^{components/script/dom/bindings/codegen/CodegenRust.py, line 1752 [r1] (raw file):}
There's a better way than descriptor.prototypeChain[-1].

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^{components/script/mem.rs, line 16 [r1] (raw file):}
Would it make sense to pass a reflector here, to clarify that we're erasing the type?

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^{components/script/mem.rs, line 23 [r1] (raw file):}
Nit: backticks around HeapSizeOf.

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^{components/script/mem.rs, line 28 [r1] (raw file):}
where should be indented four spaces, and the following brace should go on the next line, not indented.

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Comments from the review on Reviewable.io

[bors-servo]

☔ The latest upstream changes (presumably #7976) made this pull request unmergeable. Please resolve the merge conflicts.

[nox]

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

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^{components/script/mem.rs, line 16 [r1] (raw file):}
I would rather we keep the HeapSizeOf bound, and for this we need to pass T. What do you think?

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Comments from the review on Reviewable.io

[bors-servo]

☔ The latest upstream changes (presumably #7873) made this pull request unmergeable. Please resolve the merge conflicts.

[Ms2ger]

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

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Reviewed 3 of 3 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

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^{components/script/mem.rs, line 16 [r1] (raw file):}
k.

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Comments from the review on Reviewable.io

[bors-servo]

📌 Commit da67630 has been approved by Ms2ger

[bors-servo]

⌛ Testing commit da67630 with merge 5a0a91e...

[bors-servo]

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