Previously WebrtcDummyVideoEncoder was using callbacks to report
network bitrate and keyframe requests. With this change the new
NetworkStateObserver interface will be used instead. This simplifies
how the notifications are passed to the frame scheduler and also allows
the frame scheduler to receive RTT estimate if it needs to use it.

Also cleaned up threading logic in WebrtcDummyVideoEncoder and
WebrtcDummyVideoEncoderFactory.

Review-Url: https://codereview.chromium.org/2405333002
Cr-Commit-Position: refs/heads/master@{#424595}