Pass stream source as MediaStreamType in TrackControls

This CL completes a TODO in media_stream_controls.h, by switching from
std::string to MediaStreamType. Accordingly, it changes related mojo files,
renderer side UserMediaProcessor and browser side MediaStreamManager.

This will let us add new MediaStreamTypes for request in follow-up CLs.

Bug: 326740
Change-Id: I13e2613c190d342c95f084b3a06933a9458a12a8
Reviewed-on: https://chromium-review.googlesource.com/1159839
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Christian Fremerey <chfremer@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580613}
19 files changed