(Relanding) Restores larger output buffer size when output stream requiring smaller size is closed

media/audio/mac/audio_manager_mac.h

Index: media/audio/mac/audio_manager_mac.h
diff --git a/media/audio/mac/audio_manager_mac.h b/media/audio/mac/audio_manager_mac.h
index f5159e956cc44a249af38a698cf4e3399969f6b5..123d33782e2806b20e6f31616e9f306086f9a3ae 100644
--- a/media/audio/mac/audio_manager_mac.h
+++ b/media/audio/mac/audio_manager_mac.h
@@ -10,6 +10,7 @@
 #include <stddef.h>
 
 #include <list>
+#include <map>
 #include <memory>
 #include <string>
 
@@ -104,6 +105,12 @@ class MEDIA_EXPORT AudioManagerMac : public AudioManagerBase {
                              bool* size_was_changed,
                              size_t* io_buffer_frame_size);
 
+  // Called by AUHALStream::Close() before releasing the stream.
+  // This method is a special contract between the real stream and the audio
+  // manager and it ensures that we only try to increase the IO buffer size
+  // for real streams and not for fake or mocked streams.
+  void PrepareForAttemptToIncreaseIOBufferSize(AudioDeviceID device_id);
+
   // Number of constructed output and input streams.
   size_t output_streams() const { return output_streams_.size(); }
   size_t low_latency_input_streams() const {
@@ -127,6 +134,24 @@ class MEDIA_EXPORT AudioManagerMac : public AudioManagerBase {
   // sample rate has changed, otherwise does nothing.
   void HandleDeviceChanges();
 
+  // Returns true if any active input stream is using the specified |device_id|.
+  bool AudioDeviceIsUsedForInput(AudioDeviceID device_id);
+
+  // This method is called when an output stream has been released and it takes
+  // the given |device_id| and scans all active output streams that are
+  // using this id. The goal is to find a new (larger) I/O buffer size which
+  // can be applied to all active output streams since doing so will save
+  // system resources.
+  // Note that, it is only called if no input stream is also using the device.
+  // Example: two active output streams where #1 wants 1024 as buffer size but
+  // is using 256 since stream #2 wants it. Now, if stream #2 is closed down,
+  // the native I/O buffer size will be increased to 1024 instead of 256.
+  // Returns true if it was possible to increase the I/O buffer size and
+  // false otherwise.
+  // TODO(henrika): possibly extend the scheme to also take input streams into
+  // account.
+  bool IncreaseIOBufferSizeIfPossible(AudioDeviceID device_id);
+
   std::unique_ptr<AudioDeviceListenerMac> output_device_listener_;
 
   // Track the output sample-rate and the default output device
@@ -149,6 +174,20 @@ class MEDIA_EXPORT AudioManagerMac : public AudioManagerBase {
   std::list<AUAudioInputStream*> low_latency_input_streams_;
   std::list<AUHALStream*> output_streams_;
 
+  // Maps device IDs and their corresponding actual (I/O) buffer sizes for
+  // all output streams using the specific device.
+  std::map<AudioDeviceID, size_t> output_io_buffer_size_map_;
+
+  // Device ID set bu the PrepareForAttemptToIncreaseIOBufferSize() method.
+  // Only modified by real audio output stream implementations, i.e., not
+  // fake audio output streams.
+  AudioDeviceID device_id_used_by_closing_output_stream_;
+
+  // Set to true in PrepareForAttemptToIncreaseIOBufferSize() to prepare for
+  // a possible modification of the native I/O buffer size. Set to false after
+  // an attempt to increase the I/O buffer size has finalized.
+  bool try_to_increase_io_buffer_size_;
+
   DISALLOW_COPY_AND_ASSIGN(AudioManagerMac);
 };