Fix smooth scroll jump when switching scroll handling from MT to CC
To simulate position:sticky, some websites have JS code that conditionally
attaches position:fixed depending on the scroll offset. This CL finishes the
the animations on MT before switching to compositor scrolling.
ScrollingCoordinator proxys the call to clear main thread scrolling reasons to
ScrollAnimator which calls weblayer->clearMTSReasons when the scroll offset
animation is finished. ScrollAnimator adds a temporary main thread scrolling
reason until the animation is finished.
BUG=581875
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
Review URL: https://codereview.chromium.org/1648293003
Cr-Commit-Position: refs/heads/master@{#377476}
19 files changed
