Auto merge of #12751 - emilio:transitions-raf, r=pcwalton

compositor: Send animation ticks to layout even if there are script animation frames.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #12749 (github issue number if applicable).

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The script tick ends up only processing JS callbacks related to animation
frames, so CSS transitions/animations end up not working as expected.

This could have accidentally worked before #12563 because we over-restyled, but
now this is no longer the case.

Other possible way to do it is making a layout reflow with RAF handle CSS
animations/transitions too, but that may not work if the reflow ends up being
suppressed (that could very well be the case), and we'd need to handle a lot
more state in the document, so this solution (assuming it doesn't break try)
seems a bit less flacky.

Missing a test, will add one soon. Fixes #12749.

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