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The present CL is for the animation of the divider and its white handle,
when split view is activated by dragging a window, if the window is not
minimized and the dragging is not tab dragging. The animation is based
on:
https://mccanny.users.x20web.corp.google.com/www/splitscreen-motion/index.html#window-drop

The present CL deviates from the specification in an edge case as shown
in the video linked in Comment 14 here:
https://bugs.chromium.org/p/chromium/issues/detail?id=934977#c14

See how the divider moves toward the side of the screen where the window
is snapped, whereas the mockup shows the divider moving away from that
side of the screen. The specification says that the white handle shall
spawn from "2dp off centerline of divider (in direction of drop)," which
in this case is 2dp to the left of where the divider spawns from. Then
the divider would pass through the point where the handle spawns.
Because that seems wrong, where the specification says "in direction of
drop," I instead use the direction from which the divider spawns, as you
see in the video.

chrome://flags/#ui-slow-animations affects the divider but not the white
handle, because the white handle uses SlideAnimation. The flag therefore
desynchronizes the spawning animations, but fortunately, the video
referenced above opens in a player where playback speed can be adjusted.

Test: manual
Bug: 934977
Change-Id: Ide325c36c939fbf4af07b878811207c79afc206c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1601368Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659279}