Add namespace comment in mock_notification_image_retainer.h

Change-Id: I837334681a63e4d174e6bc2a9da67ef8db139db3
Reviewed-on: https://chromium-review.googlesource.com/c/1316099Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Xi Cheng <chengx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605135}

chrome/browser/notifications/win/mock_notification_image_retainer.h

@@ -10,7 +10,7 @@
 
 namespace gfx {
 class Image;
-}
+}  // namespace gfx
 
 // A mock NotificationImageRetainer class for use with unit tests. Returns
 // predictable paths to callers wanting to register temporary files.