Fix issue with conflicting behavior if setting a max BW with b=AS on both audio and video.

This reverts to previous behavior where b=AS only affects the codec bitrate for audio streams, and not the max bandwidth estimate.

BUG=chromium:703903

Review-Url: https://codereview.webrtc.org/2774123002
Cr-Original-Commit-Position: refs/heads/master@{#17386}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1ccf73f830fd3f4ca75b0e4fd15adc784991edaa
3 files changed