Reduces number of running (high-priority) threads, even though the
thread was practically blocked all the time.
Also adding DCHECKs to make sure we're not trying to use certain
sender-only methods on receivers and vice versa.
BUG=webrtc:1675, webrtc:1695
R=stefan@webrtc.org
Review URL: https://codereview.webrtc.org/1222193003
Cr-Commit-Position: refs/heads/master@{#9534}
cd4a9bd225