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This feature prevents tabs from stacking and constraining themselves to the width
of the tabstrip, instead allowing the tabstrip to scroll horizontally as a
normal scroll view would.  This is done in preparation for enabling this mode
when VoiceOver is on, to fix a bug where stacked tabs would not properly become
visible when focused by VoiceOver.

Tabs continue to be stacked by default.  A new feature flag is added to force
"unstacked" mode, and a second feature flag is added to enable unstacked mode
whenever VoiceOver is running.

BUG=979803

Change-Id: I4e4ad9f53cad1c43a0ad8d16917bd848ac2142fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833741
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#714452}