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This CL adds unit tests for CafBaseMediaRouteProvider. Some methods are
exposed for easier testing.

Also, the CL includes several minor fixes to CafBaseMRP.

Bug: 711860
Change-Id: I1675e122406a787ccdb25a32c337c50f60d71872
Reviewed-on: https://chromium-review.googlesource.com/c/1333117
Commit-Queue: Zhiqiang Zhang <zqzhang@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607811}