Add histograms for notification icons.

content/child/notifications/notification_image_loader.h

Index: content/child/notifications/notification_image_loader.h
diff --git a/content/child/notifications/notification_image_loader.h b/content/child/notifications/notification_image_loader.h
index b9818d18eeff4c1bea17d9a4979135332d560e91..37daebfaddbfa605a6375959dfc9468f2fbfec1d 100644
--- a/content/child/notifications/notification_image_loader.h
+++ b/content/child/notifications/notification_image_loader.h
@@ -14,6 +14,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/sequenced_task_runner_helpers.h"
+#include "base/time/time.h"
 #include "third_party/WebKit/public/platform/WebURLLoaderClient.h"
 
 class GURL;
@@ -99,6 +100,8 @@ class NotificationImageLoader
 
   std::vector<uint8_t> buffer_;
 
+  base::TimeTicks start_time_;
+
   DISALLOW_COPY_AND_ASSIGN(NotificationImageLoader);
 };