Use AudioStreamMonitor to control power save blocking.

media/audio/audio_output_controller.h

Index: media/audio/audio_output_controller.h
diff --git a/media/audio/audio_output_controller.h b/media/audio/audio_output_controller.h
index 0b02ee2bbb285f863774f872627d4a9bfdf27b26..f9e1033e0c34ee73caa4fbf8faf9e7eede78c5a4 100644
--- a/media/audio/audio_output_controller.h
+++ b/media/audio/audio_output_controller.h
@@ -9,6 +9,7 @@
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/timer/timer.h"
+#include "build/build_config.h"
 #include "media/audio/audio_io.h"
 #include "media/audio/audio_manager.h"
 #include "media/audio/audio_power_monitor.h"
@@ -52,11 +53,6 @@
 
 namespace media {
 
-// Only do power monitoring for non-mobile platforms that need it for the UI.
-#if !defined(OS_ANDROID) && !defined(OS_IOS)
-#define AUDIO_POWER_MONITORING
-#endif
-
 class MEDIA_EXPORT AudioOutputController
     : public base::RefCountedThreadSafe<AudioOutputController>,
       public AudioOutputStream::AudioSourceCallback,
@@ -110,6 +106,16 @@ class MEDIA_EXPORT AudioOutputController
       const AudioParameters& params, const std::string& output_device_id,
       SyncReader* sync_reader);
 
+  // Indicates whether audio power level analysis will be performed.  If false,
+  // ReadCurrentPowerAndClip() can not be called.
+  static bool will_monitor_audio_levels() {
+#if defined(OS_ANDROID) || defined(OS_IOS)
+    return false;
+#else
+    return true;
+#endif
+  }
+
   // Methods to control playback of the stream.
 
   // Starts the playback of this audio output stream.
@@ -178,6 +184,10 @@ class MEDIA_EXPORT AudioOutputController
     kError,
   };
 
+  // Time constant for AudioPowerMonitor.  See AudioPowerMonitor ctor comments
+  // for semantics.  This value was arbitrarily chosen, but seems to work well.
+  enum { kPowerMeasurementTimeConstantMillis = 10 };
+
   friend class base::RefCountedThreadSafe<AudioOutputController>;
   virtual ~AudioOutputController();
 
@@ -235,10 +245,8 @@ class MEDIA_EXPORT AudioOutputController
   // The message loop of audio manager thread that this object runs on.
   const scoped_refptr<base::SingleThreadTaskRunner> message_loop_;
 
-#if defined(AUDIO_POWER_MONITORING)
   // Scans audio samples from OnMoreData() as input to compute power levels.

[miu, 2014/09/05 18:20:25]

Not sure whether I agree with taking things this far.  Seems like a total waste
to compile this in for the unsupported platforms.  Perhaps jam@ can comment?

[DaleCurtis, 2014/09/05 18:30:14]

I was hoping the compiler would be smart enough to realize it's unused since the
static above will compile to false or true.

   AudioPowerMonitor power_monitor_;
-#endif
 
   // Flags when we've asked for a stream to start but it never did.
   base::AtomicRefCount on_more_io_data_called_;