This is better in line with Obj-C design guidelines.
BUG=None
Review-Url: https://codereview.webrtc.org/2862493003
Cr-Commit-Position: refs/heads/master@{#18023}
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}