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Since camera and microphone are particularly sensitive permissions we
should be viligent and stop streams immediately when the user revokes
the permission.

This CL uses the permission controller subscription functionality to
listen to permission changes on the relevant origins. Each DeviceRequest
will have up to 2 permission subscriptions (1 audio, 1 video). When
a permission is changed to non-Granted the specific DeviceRequest is
canceled. Subscriptions are removed when the DeviceRequest is deleted.

Because MediaStreamManager is an IO thread class but
PermissionController is a UI thread class, this CL has to do a bit of
thread hoping:

FinalizeStream(IO) > SubscribeToPermissionControllerOnUIThread(UI) >
SetPermissionSubscriptionIDs(IO) >(maybe)>
UnsubscribeFromPermissionControllerOnUIThread(UI)

DeleteRequest(IO) > UnsubscribeFromPermissionControllerOnUIThread(UI)

Bug: 1116435

Change-Id: I6649125e028d607eeb7ba5701b710a715addf6f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352790
Commit-Queue: Andy Paicu <andypaicu@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798564}