Make LoadObserver settable per video send stream. Gives client flexibility and makes the implementation slightly simpler. See discussion in: https://codereview.webrtc.org/1269863005/

BUG=webrtc:4690

Review URL: https://codereview.webrtc.org/1325263002

Cr-Commit-Position: refs/heads/master@{#9891}
8 files changed