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This is an initial cl for back gesture affordance. Many other issues
will be solved in following up cls. e.g, fling event support;
make affordance can be moved on y-axis movement; affordance should not
under frame area, etc.

Recorded video:
https://drive.google.com/file/d/1p7T-RiHU8nhaKmyAgrqA_bqITkCDBi7p/view?usp=sharing

Bug: 1002733
Change-Id: I89c02372babf2d562e56717c7966df2e8e5405f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834826
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702635}