Bug 451053. Move nsMutationGuard into a shared location and use it to assert some invariants. r+sr=sicking

content/base/src/nsGenericElement.cpp

@@ -1701,9 +1701,6 @@ nsStaticContentList::Item(PRUint32 aIndex, nsIDOMNode** aReturn)
 }
 
 //----------------------------------------------------------------------
-
-PRUint32 nsMutationGuard::sMutationCount = 0;
-
 nsGenericElement::nsDOMSlots::nsDOMSlots(PtrBits aFlags)
   : nsINode::nsSlots(aFlags),
     mBindingParent(nsnull)
@@ -5162,6 +5159,10 @@ TryMatchingElementsInSubtree(nsINode* aRoot,
   nsIContent * const * kidSlot = aRoot->GetChildArray();
   nsIContent * const * end = kidSlot + count;
 
+#ifdef DEBUG
+  nsMutationGuard debugMutationGuard;
+#endif
+  
   PRBool continueIteration = PR_TRUE;
   for (; kidSlot != end; ++kidSlot) {
     nsIContent* kid = *kidSlot;
@@ -5223,6 +5224,11 @@ TryMatchingElementsInSubtree(nsINode* aRoot,
     /* Make sure to clean this up */
     prevSibling->~RuleProcessorData();
   }
+
+#ifdef DEBUG
+  NS_ASSERTION(!debugMutationGuard.Mutated(0), "Unexpected mutations happened");
+#endif
+
   return continueIteration;
 }