Add default impl of WebMediaStreamCenter::getMediaStreamTrackSources.

This is to prepare for removal of that function. The path for getSources has recently moved to UserMediaClient and this function is obsolete.

After this CL, the implementation will be removed from Chromium, then the function will be removed from blink.

BUG=420975

Review URL: https://codereview.chromium.org/635493002

git-svn-id: svn://svn.chromium.org/blink/trunk@183343 bbb929c8-8fbe-4397-9dbb-9b2b20218538