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Place the colored monogram inside a gray circle. This matches the
fallback style on mobile.

Screenshots:
https://screenshot.googleplex.com/SjGT71VeutN.png
https://screenshot.googleplex.com/6eXirCU0knX.png
https://screenshot.googleplex.com/8xcd9KZvvgp.png

Bug: 905437
Change-Id: Ib26c04fc22e4eb50685c5de04300dd543a625cee
Reviewed-on: https://chromium-review.googlesource.com/c/1354318
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613194}