Auto merge of #15943 - hiikezoe:animation-on-pseudo, r=heycam
Auto merge of #15943 - hiikezoe:animation-on-pseudo, r=heycam

Use computed styles of the pseudo and its parent when calling Gecko_U…

…pdateAnimations to update CSS Animations on pseudo-elements.

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This is a PR of https://bugzilla.mozilla.org/show_bug.cgi?id=1346408 .

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- [X] These changes do not require tests because it's for stylo

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