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This gives users a way to reach apps when the overflow is beyond filled
on tablet mode. Also fix a bug with the shelf layout manager handling
events on the overflow.

Test: manual
Bug: 808237
Change-Id: Ia3d4a14c8c1a15f7c991a201bdad94d6657f4e6d
Reviewed-on: https://chromium-review.googlesource.com/1208826Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589221}