Persist the user's active audio device choice across chromeos session and reboots.

chromeos/dbus/cras_audio_client.h

Index: chromeos/dbus/cras_audio_client.h
diff --git a/chromeos/dbus/cras_audio_client.h b/chromeos/dbus/cras_audio_client.h
index 0eff85d66203e46469ace78d5022aaf3df4831de..dd730403bbfd873d840891ebec20e233fbf11c88 100644
--- a/chromeos/dbus/cras_audio_client.h
+++ b/chromeos/dbus/cras_audio_client.h
@@ -70,6 +70,9 @@ class CHROMEOS_EXPORT CrasAudioClient : public DBusClient {
   // contains the detailed dbus error message.
   typedef base::Callback<void(const std::string&,
                               const std::string&)> ErrorCallback;
+  // A callback for cras dbus method WaitForServiceToBeAvailable.
+  typedef base::Callback<void(bool service_is_ready)>
+      WaitForServiceToBeAvailableCallback;
 
   // Gets the volume state, asynchronously.
   virtual void GetVolumeState(const GetVolumeStateCallback& callback) = 0;
@@ -125,6 +128,10 @@ class CHROMEOS_EXPORT CrasAudioClient : public DBusClient {
   // |node_id|.
   virtual void SwapLeftRight(uint64_t node_id, bool swap) = 0;
 
+  // Runs the callback as soon as the service becomes available.
+  virtual void WaitForServiceToBeAvailable(
+      const WaitForServiceToBeAvailableCallback& callback) = 0;
+
   // Creates the instance.
   static CrasAudioClient* Create();