Added NS_IMPL_AGGREGATED_QUERY_HEAD macro.

xpcom/base/nsAgg.h

@@ -123,4 +123,14 @@ _class::Internal::Release(void)                                             \
     return agg->mRefCnt;                                                    \
 }                                                                           \
 
+// for use with QI macros in nsISupportsUtils.h:
+
+#define NS_IMPL_AGGREGATED_QUERY_HEAD(_class)                                              \
+NS_IMETHODIMP _class::AggregatedQueryInterface(REFNSIID aIID, void** aInstancePtr)         \
+{                                                                                          \
+  NS_ASSERTION(aInstancePtr, "AggregatedQueryInterface requires a non-NULL destination!"); \
+  if ( !aInstancePtr )                                                                     \
+    return NS_ERROR_NULL_POINTER;                                                          \
+  nsISupports* foundInterface;
+
 #endif /* nsAgg_h___ */