Bug 721069. Remove default case from filter switch statement. r=cjones

This fixes a clang warning and allows us to get warnings when a new enumeration is added to the enum.

ipc/glue/IPCMessageUtils.h

@@ -484,10 +484,8 @@ struct ParamTraits<mozilla::GraphicsFilterType>
       WriteParam(msg, int32(param));
       return;
 
-    default:
-      NS_RUNTIMEABORT("not reached");
-      return;
     }
+    NS_RUNTIMEABORT("not reached");
   }
 
   static bool Read(const Message* msg, void** iter, paramType* result)