Update bitrate adjustment with true buffer used size.

This used to be updated with the reserved capacity of the buffer,
not the actual portion in use.

BUG=webrtc:6034

Review-Url: https://codereview.webrtc.org/2620653005
Cr-Original-Commit-Position: refs/heads/master@{#15982}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a3f6ce10ed3863151e2cc7142c492394efda8d20
1 file changed