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This patch adds a checkmark which is at the left of the currenly playing device on the cast menu.

BUG=none
TEST=manually tested

Review URL: https://codereview.chromium.org/456653002

Cr-Commit-Position: refs/heads/master@{#288325}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288325 0039d316-1c4b-4281-b951-d872f2087c98