GitLab

This CL adds user actions for pressing the home button and the back
button.

Bug: 1018916
Change-Id: I0d97812cbe53a0334dff7c32bbe016bd82d41c8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900389Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713570}