Adding labels to Duet

Adding text labels to the bottom toolbar buttons. This also adds a
chrome://flags flag.

Bug: 965556

Change-Id: Ic001014dc525036972e139c826da147aa8256dcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1552152
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661907}
29 files changed