Fix MediaObserver notifications in MediaStreamManager.

This CL fixes the stream type used to notify MediaObserver about
cancelled MediaStream requests.

Before this CL, NUM_MEDIA_TYPES was used as stream type to indicate
that all stream types should be cancelled.
However, the MediaObserver end does not interpret NUM_MEDIA_TYPES this
way and the request to update the UI is ignored.

This CL sends a separate notification for each stream type so that the
UI actually gets updated for all stream types in use.

Bug: 816033
Change-Id: Ib7d3b3046d1dd0976627f8ab38abf086eacc9405
Reviewed-on: https://chromium-review.googlesource.com/939630
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540122}
  ~DeviceRequest() { RunMojoCallbacks(); }

  ~DeviceRequest() { RunMojoCallbacks(); }
