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This CL adds a circular colored background to dismiss buttons in the
GMC dialog. This fixes a contrast issue where the X was not always
visible against the media artwork.

Bug: 1006049
Change-Id: Ib56353437fce6e4ec1ad8e15268e5bbbb284607c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829537Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701611}