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Avoids having additional high-frequency post tasking for no-op tasks in
the background.

To keep track of when to stop animating without an AnimationEnded
callback, we keep track of the last-drawn animation state. This also has
the side effect of allowing us to avoid repainting the loading-progress
bar if would paint the same pixels we drew last time. This should avoid
a lot of repaints for slow-loading sites.

This also ends up fixing backwards-jumping indicators. Probably because
the logic for clamping the loading-progress timer got easier.

Bug: chromium:907044, chromium:905020, chromium:905452
Change-Id: Ic83748bad83115ef2283c881a89ed65e3d3938ab
Reviewed-on: https://chromium-review.googlesource.com/c/1346831Reviewed-by: Sidney San Martín <sdy@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611234}