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This CL adds new material design icon to ARC notification.
This CL also removes notification buttons on initial notification
based on UX review (https://crbug.com/726241#c20).

The icon and the accent color are effective only when
chrome://flags/#enable-message-center-new-style-notification
is Enabled (which is ToT default).

TEST=manual
BUG=726241

Change-Id: Id617ad8fa8ab42f5494cf53fd27100397a09ceee
Reviewed-on: https://chromium-review.googlesource.com/662837Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502165}