GitLab

A few changes:
 - Catches some hotseat corner cases causing multiple state changes
 - adds tests to catch these multi-state changes
 - Adds the ability to determine that Overview is in the process of
   being toggled. Before this cl, overview would be in the process
   of being shown but it was not clear if it was being shown or hidden
   until it was too late.

Bug: 1018346
Change-Id: I6e8dfd0a9b0256134770e207345f4926bfc512ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1885173
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710555}