Bug 1089417 patch 2 - Add assertion that RecordStyleSheetChange is called inside an update. r=heycam

This was just something that seemed worth asserting in the process of
debugging, since I wanted to see if it was the problem.

layout/base/nsPresShell.cpp

@@ -4728,6 +4728,9 @@ nsIPresShell::ReconstructStyleDataExternal()
 void
 PresShell::RecordStyleSheetChange(nsIStyleSheet* aStyleSheet)
 {
+  // too bad we can't check that the update is UPDATE_STYLE
+  NS_ASSERTION(mUpdateCount != 0, "must be in an update");
+
   if (mStylesHaveChanged)
     return;