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It's possible the new UX won't have UX approval by the M80 branch point, so
place all the changes behind a feature flag, just to be safe.

Bug: 1020423
Change-Id: Ia10280e3313fc5754c0c0ac46a33db59f98cf7f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1931646Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719236}