Use the alternate notification UI for Mac

UX preferred this one.

BUG=571056

Review URL: https://codereview.chromium.org/1618713003

Cr-Commit-Position: refs/heads/master@{#370695}

chrome/browser/notifications/notification_ui_manager_mac.mm

@@ -119,10 +119,11 @@ void NotificationUIManagerMac::Add(const Notification& notification,
   // TODO(miguelg): Implement support for buttons
   toast.get().hasActionButton = NO;
 
-  // Some functionality is only available in 10.9+
+  // Some functionality is only available in 10.9+ or requires private APIs
   // Icon
-  if ([toast respondsToSelector:@selector(setContentImage:)]) {
-    [toast setValue:notification.icon().ToNSImage() forKey:@"contentImage"];
+  if ([toast respondsToSelector:@selector(_identityImage)]) {
+    [toast setValue:notification.icon().ToNSImage() forKey:@"_identityImage"];
+    [toast setValue:@NO forKey:@"_identityImageHasBorder"];
   }
 
   // Tag