Fix so that codec max bitrate doesn't override.

Currently the codec specific max bitrate that is set in the SDP
gets overridden by the value set with the "b=AS" attribute
(WebRtcVideoChannel::SetSendParameters). But at the
WebRtcVideoSendStream level it does the opposite - the codec
specific max bitrate value overrides the values that could be
set by RtpParameters or the "b=AS" value
(in WebRtcVideoSendStream::CreateVideoEncoderConfig). This change
updates the logic to be consistent with what happens at the
WebRtcVideoChannel level, and allows the RtpParameter max bitrate
to override the codec specific max bitrate.

Bug: webrtc:8655
Change-Id: I3f0347cb7cffcfc577484231b061ab0712453e69
Reviewed-on: https://webrtc-review.googlesource.com/88520
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23989}