GitLab

Clusterfuzz identified a race between the media thread calling SetVolume
and the audio device thread calling ProvideInput. Add a lock to
synchronize access to |volume_| between threads.

Also adds some thread_checkers to just firm up the contract that the
majority of these methods are called only by the media thread.

BUG=588992

Committed: https://crrev.com/57c6958a99f2c62a4408d2a9d262a22129309106
Cr-Commit-Position: refs/heads/master@{#379349}

Review URL: https://codereview.chromium.org/1748183006

Cr-Commit-Position: refs/heads/master@{#381763}