Introduces a new capture class derived from cricket::VideoCapturer that
provides the ability to switch cameras and updates AppRTCDemo to use it.
Some future work pending to clean up AppRTCDemo UI.
BUG=4070
R=magjed@webrtc.org
Review URL: https://webrtc-codereview.appspot.com/48279005
Cr-Commit-Position: refs/heads/master@{#9137}