Pass timestamps to VideoAdapter instead of setting expected input frame rate, and use that to calculate when frames should be dropped. BUG=webrtc:4938 TEST=Enable quality slider and HUD in debug settings. Request low fps with the quality slider and observe dropped frames. Review-Url: https://codereview.webrtc.org/1982983003 Cr-Commit-Position: refs/heads/master@{#12811}