[Media Galleries] Lazily initialize the storage monitor.

chrome/browser/media_galleries/media_galleries_preferences.h

Index: chrome/browser/media_galleries/media_galleries_preferences.h
diff --git a/chrome/browser/media_galleries/media_galleries_preferences.h b/chrome/browser/media_galleries/media_galleries_preferences.h
index 6ece27dffc047363f4b61e69913ae0baceb0824e..b208de39bcb27f506725140456355dd1c3132684 100644
--- a/chrome/browser/media_galleries/media_galleries_preferences.h
+++ b/chrome/browser/media_galleries/media_galleries_preferences.h
@@ -115,14 +115,19 @@ class MediaGalleriesPreferences : public ProfileKeyedService,
                                   public RemovableStorageObserver {
  public:
   class GalleryChangeObserver {
-    public:
-     // |extension_id| specifies the extension affected by this change.
-     // It is empty if the gallery change affects all extensions.
-     virtual void OnGalleryChanged(MediaGalleriesPreferences* pref,
-                                   const std::string& extension_id) {}
-
-    protected:
-     virtual ~GalleryChangeObserver();
+   public:
+    // |extension_id| specifies the extension affected by this change.
+    // It is empty if the gallery change affects all extensions.
+    virtual void OnGalleryChanged(MediaGalleriesPreferences* pref,
+                                  const std::string& extension_id) {}
+
+   protected:
+    virtual ~GalleryChangeObserver();
+  };
+
+  class Vendor {
+   public:
+    virtual MediaGalleriesPreferences* GetPreferences(Profile* profile) = 0;
   };
 
   explicit MediaGalleriesPreferences(Profile* profile);