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This allows MediaStreamTrack::getSources to go through UserMediaClient.

To land after this:
Chrome CL part 1: https://codereview.chromium.org/562643003/
Blink CL part 2: https://codereview.chromium.org/559423002
Chrome CL part 2 (TBD)

BUG=406094

Review URL: https://codereview.chromium.org/560843002

git-svn-id: svn://svn.chromium.org/blink/trunk@181822 bbb929c8-8fbe-4397-9dbb-9b2b20218538