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This was a rgression caused by a previous CL of mine https://crrev.com/c/1865543.
Using the developer keyboard shortcut to enable tablet mode should
force the overview tray button visible even though the internal
input events are not being blocked.

BUG=1016594, 925087
TEST=Manually, added a new test.

Change-Id: Ic9046c07dcce9de03c5fc5acd10a2077d599c711
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872468Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708222}