Boost WaveOut thread priority for background audio.

XP seems to be lowering the priority of the thread responding to
audio callbacks when backgrounded, causing glitching.

BUG=161307
TEST=background playback on XP works without issue.

Review URL: https://codereview.chromium.org/11316284

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@170870 0039d316-1c4b-4281-b951-d872f2087c98

media/audio/win/waveout_output_win.cc

@@ -11,6 +11,8 @@
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
+#include "base/message_loop_proxy.h"
+#include "base/threading/platform_thread.h"
 #include "media/audio/audio_io.h"
 #include "media/audio/audio_util.h"
 #include "media/audio/win/audio_manager_win.h"
@@ -108,6 +110,11 @@ PCMWaveOutAudioOutputStream::PCMWaveOutAudioOutputStream(
   }
   format_.SubFormat = KSDATAFORMAT_SUBTYPE_PCM;
   format_.Samples.wValidBitsPerSample = params.bits_per_sample();
+
+  // Boost thread priority.  Required for glitch free background audio.
+  DCHECK(manager_->GetMessageLoop()->BelongsToCurrentThread());
+  base::PlatformThread::SetThreadPriority(
+      GetCurrentThread(), base::kThreadPriority_RealtimeAudio);
 }
 
 PCMWaveOutAudioOutputStream::~PCMWaveOutAudioOutputStream() {