This fixes a problem where "b=AS" and "x-google-start-bitrate" can't
be used together. It also starts taking the minimum of "b=AS" and
"x-google-max-bitrate", instead of just letting "b=AS" win.
BUG=webrtc:5811
R=pbos@webrtc.org
Review URL: https://codereview.webrtc.org/1904063003 .
Cr-Commit-Position: refs/heads/master@{#12519}