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Cast SDK uses some special strings (auto-join, cast-session) for presentation ID that may not correspond to that of existing routes.
This CL handles those cases and sends the requests to the extension MRP.

Bug: 786654
Change-Id: Ib32befe63936473edd71d16959e7a8caf8db81a9
Reviewed-on: https://chromium-review.googlesource.com/780584
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519818}