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This CL adds support for horizontal swipe-to-dismiss gestures, adds
fling support and updates the existing motion specs. It also adds unit
tests for the mediator.

Flings currently don't affect the speed of the resulting animations as
it turned out to be complicated to implement.

There are several cases that are handled in this CL:
- No fling:
  - If the message is within the dismiss threshold, vertical or
    horizontal, the message is animated back to the idle position, i.e.
    center.
  - If it's outside the threshold, the message is dismissed by being
    animated to out of screen.
- Fling:
  - If the message is flung toward the center from outside the dismiss
    threshold, it is animated to the idle position.
  - If it's flung from within the threshold to the outside, the message
    is dismissed with animation.
  - If it's flung toward out of screen from outside the dismiss
    threshold, the message is dismissed with animation.

Bug: 1157213
Change-Id: I82cfac7111e5ada8687be74a90162784a1e7c992
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2602706
Commit-Queue: Sinan Sahin <sinansahin@google.com>
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843290}