GitLab

Following r478084, the main thread may contend with the audio thread
on accessing AudioBufferSourceNodeHandler's mutable state. Coordinate
such access by introducing a Mutex over |min_playback_rate_|.

Using atomic ops would be the natural choice for handling this, but
steering clear of those over doubles (cf. https://crrev.com/1256053006)
until std::atomic<> is allowed.

R=haraken,hongchan
BUG=731568

Review-Url: https://codereview.chromium.org/2929283002
Cr-Commit-Position: refs/heads/master@{#478679}