Issue #2452 - Prevent nsDocShell access when it is already being destroyed

https://bugzilla.mozilla.org/show_bug.cgi?id=1432396

dom/base/nsContentSink.h

@@ -304,6 +304,8 @@ protected:
   // True if this is parser is a fragment parser or an HTML DOMParser.
   // XML DOMParser leaves this to false for now!
   uint8_t mRunsToCompletion : 1;
+  // True if we are blocking load event.
+  bool mIsBlockingOnload : 1;
   
   //
   // -- Can interrupt parsing members --