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In the new overview layout, scrolling is needed to be able to see each
available window past the sixth window.

R=sammiequon@chromium.org, xdai@chromium.org

Bug: 981174
Change-Id: Idc207bf155e8458231114344650837a5783c8854
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1709876Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Danton Vu <dantonvu@google.com>
Cr-Commit-Position: refs/heads/master@{#684035}