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This will be used for in-browser DIAL MRP
(https://chromium-review.googlesource.com/c/chromium/src/+/1036611) and
other future MRP implementations to indicate a MediaRoute already
exists when trying to CreateRoute.

Bug: 816628
Change-Id: Id43d91be7318362043cc33a3c0500cc9463f5a5d
Reviewed-on: https://chromium-review.googlesource.com/1062145Reviewed-by: Adam Parker <amp@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Derek Cheng <imcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559249}