[Media Galleries] Lazily initialize the storage monitor.

chrome/browser/media_galleries/media_file_system_registry.h

Index: chrome/browser/media_galleries/media_file_system_registry.h
diff --git a/chrome/browser/media_galleries/media_file_system_registry.h b/chrome/browser/media_galleries/media_file_system_registry.h
index f12010a3277956b57c73a22464226734f5b61e12..9acdf5241106014f833f4aefbf457d9e69e908f5 100644
--- a/chrome/browser/media_galleries/media_file_system_registry.h
+++ b/chrome/browser/media_galleries/media_file_system_registry.h
@@ -82,6 +82,8 @@ class MediaFileSystemRegistry : public RemovableStorageObserver {
   // MediaGalleriesPreferences directly because this method also ensures that
   // currently attached removable devices are added to the preferences.
   // Called on the UI thread.
+  // Note: Caller must ensure that the storage monitor is initialized before
+  // calling this method.
   MediaGalleriesPreferences* GetPreferences(Profile* profile);
 
   // RemovableStorageObserver implementation.