reland http://codereview.chromium.org/10830063/

reland http://codereview.chromium.org/10830063/

refactor EnumerateDevices to make it a persistent request.
After the requester calls EnumerateDevices, it should expect new device list as initial response and whenever devices are changed.
A new method StopEnumerateDevices is also added to allow the request to stop device enumeration.

It also includes corresponding change in Pepper since EnumerateDevices contract is changed (http://codereview.chromium.org/10837064/).
This patch keeps current Pepper API, while using the new API of EnumerateDevices from MediaStreamDispatcher. 
Pepper calls StopEnumerateDevices when it receives the first enumerated result. 

A new Pepper API will be created in a separate patch to make EnumerateDevices persistent. 

BUG=137799
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=150803