If we don't reconfigure capture session in a single transaction,
RTCCameraPreviewView goes transparent when switching cameras. This is
undesired behavior.
BUG=webrtc:7177
Review-Url: https://codereview.webrtc.org/2811643006
Cr-Commit-Position: refs/heads/master@{#17664}
Implements a new capturer based on the new video source design.
BUG=webrtc:7177
Review-Url: https://codereview.webrtc.org/2776703002
Cr-Commit-Position: refs/heads/master@{#17486}
