Implement createCaption and deleteCaption on HTMLTableElement

[dzbarsky]

[Ms2ger]

Make tests/wpt/web-platform-tests/html/semantics/tabular-data/the-caption-element/caption_001.html test that the new caption is indeed the first child.

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Reviewed 7 of 7 files at r1.
Review status: all files reviewed at latest revision, 5 unresolved discussions, all commit checks successful.

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^{components/script/dom/htmltableelement.rs, line 88 [r1] (raw file):}
Maybe a little too clever; feel free to use a local variable for the first child.

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^{tests/wpt/web-platform-tests/html/semantics/tabular-data/the-table-element/caption-methods.html, line 15 [r1] (raw file):}
This is HTML, so foo:caption isn't a particularly interesting case. You'll need some dynamically generated tables.

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^{tests/wpt/web-platform-tests/html/semantics/tabular-data/the-table-element/caption-methods.html, line 54 [r1] (raw file):}
And assert_equals(testCaption, table0.firstChilfd)

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^{tests/wpt/web-platform-tests/html/semantics/tabular-data/the-table-element/caption-methods.html, line 84 [r1] (raw file):}
Come again?

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^{tests/wpt/web-platform-tests/html/semantics/tabular-data/the-table-element/caption-methods.html, line 86 [r1] (raw file):}
For bonus points, add a (dynamic) test for createCaption() where the table element has a prefix; the newly created caption should not have a prefix. (Gecko should fail this, I believe.)

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Comments from the review on Reviewable.io

[Ms2ger]

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

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Review status: all files reviewed at latest revision, 5 unresolved discussions, all commit checks successful.

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Comments from the review on Reviewable.io

[Ms2ger]

Squash?

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

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Reviewed 2 of 2 files at r2.
Review status: all files reviewed at latest revision, 2 unresolved discussions, all commit checks successful.

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Comments from the review on Reviewable.io

[Ms2ger]

@bors-servo r+

[bors-servo]

📌 Commit e24a867 has been approved by Ms2ger

[bors-servo]

⌛ Testing commit e24a867 with merge f41834e...

[bors-servo]

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