Only increment scale counter and adaptation stats (and store last_adaptation_request_) if sink_wants_ is updated.

BUG=webrtc:7492

Review-Url: https://codereview.webrtc.org/2800403002
Cr-Original-Commit-Position: refs/heads/master@{#17808}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d0de295119a32fc994b18e49e1629d88a9d4e386