Add functionality to get default media device IDs from user preferences.

chrome/browser/media/webrtc/media_capture_devices_dispatcher.h

Index: chrome/browser/media/webrtc/media_capture_devices_dispatcher.h
diff --git a/chrome/browser/media/webrtc/media_capture_devices_dispatcher.h b/chrome/browser/media/webrtc/media_capture_devices_dispatcher.h
index 2bd48511209467511055abc9d407558fb25f5afa..da7aba46dccf980f9f07d9a597493574466690c5 100644
--- a/chrome/browser/media/webrtc/media_capture_devices_dispatcher.h
+++ b/chrome/browser/media/webrtc/media_capture_devices_dispatcher.h
@@ -9,6 +9,7 @@
 #include <list>
 #include <map>
 #include <memory>
+#include <string>
 
 #include "base/callback.h"
 #include "base/macros.h"
@@ -109,6 +110,14 @@ class MediaCaptureDevicesDispatcher : public content::MediaObserver {
                                    bool video,
                                    content::MediaStreamDevices* devices);
 
+  // Helper to get default device IDs. If the returned value is an empty string,
+  // it means that there is no default device for the given device |type|. The
+  // only supported |type| values are content::MEDIA_DEVICE_AUDIO_CAPTURE and
+  // content::MEDIA_DEVICE_VIDEO_CAPTURE.
+  // Must be called on the UI thread.
+  std::string GetDefaultDeviceIDForProfile(Profile* profile,
+                                           content::MediaStreamType type);
+
   // Helpers for picking particular requested devices, identified by raw id.
   // If the device requested is not available it will return NULL.
   const content::MediaStreamDevice*