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This is the chromium side of the effort to consolidate
TestMediaResourceTracker definitions. Currently there are two
definitions: one in chromium, and one in the internal cast codebase.
After this CL and an internal CL, there will be only one version (the
chromium version).

Test: Ran cast_media_unittests locally.

Bug: b/143154455
Change-Id: I16d6a27679a395a0d4ae512cf29a7fe98e08af84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1975071Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Antonio Rivera <antoniori@google.com>
Cr-Commit-Position: refs/heads/master@{#728631}