Message Center: Add flag to chrome://flags for enabling redesigned stacking bar UI.

BUG=921250

Change-Id: I09ebc0d4d3acc931d5d3ad28152f5040c83bfca6
Reviewed-on: https://chromium-review.googlesource.com/c/1492834
Commit-Queue: Tim Song <tengs@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636682}
5 files changed