Simplify content::VideoCaptureImpl to support only a single client.
content::VideoCaptureImpl currently can support multiple
media::VideoCapture::EventHandlers, but this complexity is unnecessary.
See ref'd issue for more discussion.
BUG=299394
TEST=If creating a JS stream with getUserMedia() and then duplicating via stream(stream), a new VCSessionId should be created [1], so the 1:1 mapping from VCSessionId to VCImpl will be maintained.
[1] https://code.google.com/p/chromium/codesearch#chromium/src/content/browser/renderer_host/media/video_capture_manager.cc&type=cs&sq=package:chromium&l=222&rcl=
1439335533
Review URL: https://codereview.chromium.org/
1272323004
Cr-Commit-Position: refs/heads/master@{#344314}