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This CL adds a circular background to the Zoom icons (on top of
the navigation_bubble_shadow). The circular background supports
Dark mode (is gray when Dark mode is enabled, and white when not).

Bug: 656015
Change-Id: Ibf91c607add40be77807b88f66a7ed82f2955266
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1959031
Commit-Queue: Finnur Thorarinsson <finnur@chromium.org>
Auto-Submit: Finnur Thorarinsson <finnur@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723392}