Report dropped frames from codec thread.
Report dropped frames from codec thread.

Removes data race inside QualityScaler which were called both from the
codec thread and the invoking thread at the same time.

BUG=webrtc:5678
R=glaznev@webrtc.org

Review URL: https://codereview.webrtc.org/1919883002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#12517}

ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc