app_shell: Use real audio preferences and save them to local state

This allows app_shell to support the chrome.audio extension API in the same
way that Chrome does.

* Introduce "local state" for device wide preferences
* Use a real AudioDevicesPrefHandlerImpl in app_shell

This also sets the default output volume for app_shell (for non-HDMI output)
to 75%, which is consistent with Chrome on Chrome OS.

BUG=442401
TEST=chromeos_unittests and app_shell_unittests for audio
TBR=yoz@chromium.org for adding a test data file to extensions/test/data

Review URL: https://codereview.chromium.org/816793002

Cr-Commit-Position: refs/heads/master@{#309138}

1 files changed, 4 insertions, 0 deletions

diff --git a/chromeos/audio/audio_devices_pref_handler.h b/chromeos/audio/audio_devices_pref_handler.h
index 53a3fce..aeb7b3d 100644
--- a/chromeos/audio/audio_devices_pref_handler.h
+++ b/chromeos/audio/audio_devices_pref_handler.h
@@ -22,6 +22,10 @@ struct AudioDevice;
 class CHROMEOS_EXPORT AudioDevicesPrefHandler
     : public base::RefCountedThreadSafe<AudioDevicesPrefHandler> {
  public:
+  // Integer because C++ does not allow static const double in header files.
+  static const int kDefaultOutputVolumePercent = 75;
+  static const int kDefaultHdmiOutputVolumePercent = 100;
+
   // Gets the audio output volume value from prefs for a device. Since we can
   // only have either a gain or a volume for a device (depending on whether it
   // is input or output), we don't really care which value it is.