Bug 1535384 part 1. Eliminate some easy cases of MOZ_CAN_RUN_SCRIPT_BOUNDARY on webidl callbacks. r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D23771

dom/base/DOMIntersectionObserver.cpp

@@ -460,7 +460,8 @@ void DOMIntersectionObserver::Notify() {
     }
   }
   mQueuedEntries.Clear();
-  mCallback->Call(this, entries, *this);
+  RefPtr<dom::IntersectionCallback> callback(mCallback);
+  callback->Call(this, entries, *this);
 }
 
 }  // namespace dom