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These were part of [1], but made the diff difficult to read so moved
them to a separate cl. Also renamed StartAnimationObserver to
EnterAnimationObserver as it makes more sense in the context.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1564881

Test: added tests
Bug: 914147
Change-Id: Iaf878bf2255d3c5ac585d52fc32cdf9f15cf8029
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566361
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650504}