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Previously a single object MediaStreamUIController was used to control UI for
all streams. Replaced it with a per-stream MediaStreamUIProxy that simplifies
code in many places.
Also moved media request queueing logic from content layer to chrome. Now
different types of requests may be queued differently (e.g. there is no
reason to block screen capture requests on webcam infobar).

This change was previously landed in 197222 and reverted in 197242

TBR=vrk@chromium.org

Review URL: https://codereview.chromium.org/16342002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@204044 0039d316-1c4b-4281-b951-d872f2087c98