Bug 1675868 - Build fix. a=bustage

dom/messagechannel/MessagePortParent.h

@@ -18,7 +18,7 @@ class MessagePortService;
 
 class MessagePortParent final
     : public PMessagePortParent,
-      public SupportsWeakPtr,
+      public SupportsWeakPtr<MessagePortParent>,
       public SupportsCheckedUnsafePtr<CheckIf<DiagnosticAssertEnabled>> {
   friend class PMessagePortParent;