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Create a new class use-theme for the most-visited iframe to
differentiate whether is custom-background or use theme. Apply pill
under MD icons text only on when using theme but not for the
custom-background.

Screenshot:
https://screenshot.googleplex.com/Et8oFFtMgXb.png
https://screenshot.googleplex.com/ix4meSKVW5W.png

Bugs: 850718
Change-Id: I9416a44d9e2fc3dfd64fcf4198139d1f388ae9c3
Reviewed-on: https://chromium-review.googlesource.com/1156062
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579649}