Adding syncrhonization to PulseAudioOutputStream::SetVolume

to avoid a race with rendering callback (https://cs.chromium.org/chromium/src/media/audio/pulse/pulse_output.cc?sq=package:chromium&rcl=1475536681&l=148)

BUG=285242

Review-Url: https://codereview.chromium.org/2394483002
Cr-Commit-Position: refs/heads/master@{#422845}

media/audio/pulse/pulse_output.cc

@@ -227,6 +227,9 @@ void PulseAudioOutputStream::Stop() {
 void PulseAudioOutputStream::SetVolume(double volume) {
   DCHECK(thread_checker_.CalledOnValidThread());
 
+  // Waiting for the main loop lock will ensure outstanding callbacks have
+  // completed and |volume_| is not accessed from them.
+  AutoPulseLock auto_lock(pa_mainloop_);
   volume_ = static_cast<float>(volume);
 }