Previously probing bitrate was capped at 10Mbps, which is too low for some
application. Now ProbeContoller limits max probing rate to max allowed
bitrate, which can be specified by the application.
BUG=webrtc:6332
Review-Url: https://codereview.webrtc.org/2430133005
Cr-Commit-Position: refs/heads/master@{#14927}