content: convert zoom notifications to observer usage

content/public/browser/notification_types.h

Index: content/public/browser/notification_types.h
diff --git a/content/public/browser/notification_types.h b/content/public/browser/notification_types.h
index ced9dcfc6f7ef6dbefc39913359c3b90c6ff328a..4c3bd7153e5722169a89ea6c6835aeb6993d8bb6 100644
--- a/content/public/browser/notification_types.h
+++ b/content/public/browser/notification_types.h
@@ -336,11 +336,6 @@ enum NotificationType {
   // The source is a NavigationController.
   NOTIFICATION_REPOST_WARNING_SHOWN,
 
-  // Sent when the zoom level changes. The source is the HostZoomMap.  The
-  // details is a string of the hostname for which the zoom changed.  In case
-  // of a temporary zoom level change, the details is an empty string.
-  NOTIFICATION_ZOOM_LEVEL_CHANGED,
-
   // Custom notifications used by the embedder should start from here.
   NOTIFICATION_CONTENT_END,
 };