Bug 723446. Be a little more careful changing declarations on keyframe rules. r=dbaron

layout/style/nsCSSRules.cpp

@@ -1964,7 +1964,11 @@ nsCSSKeyframeRule::GetStyle(nsIDOMCSSStyleDeclaration** aStyle)
 void
 nsCSSKeyframeRule::ChangeDeclaration(css::Declaration* aDeclaration)
 {
-  mDeclaration = aDeclaration;
+  // Be careful to not assign to an nsAutoPtr if we would be assigning
+  // the thing it already holds.
+  if (aDeclaration != mDeclaration) {
+    mDeclaration = aDeclaration;
+  }
 
   if (mSheet) {
     mSheet->SetModifiedByChildRule();

xpcom/base/nsAutoPtr.h

@@ -66,6 +66,7 @@ class nsAutoPtr
       void
       assign( T* newPtr )
         {
+          NS_ABORT_IF_FALSE(mRawPtr != newPtr || !newPtr, "This makes no sense!");
           T* oldPtr = mRawPtr;
           mRawPtr = newPtr;
           delete oldPtr;