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This CL fixes an issue where all Cast devices show "source not
supported" after selecting a local file to cast.

Bug: 1078448, b/155777239
Change-Id: Ia71a0cca55c45ed9578e7d49ffccef7333b3319e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210634Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: John Williams <jrw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775148}