Back out bug 1323207, part 2 - Assert early if we're painting at various points we enter JS (a=backout)

This reverts commit cc6bc31c9f3fb10877292b31c1cf26927856a837.

MozReview-Commit-ID: Dljoh4lYVZ9

image/ScriptedNotificationObserver.cpp

@@ -31,9 +31,6 @@ ScriptedNotificationObserver::Notify(imgIRequest* aRequest,
                                      int32_t aType,
                                      const nsIntRect* /*aUnused*/)
 {
-  MOZ_RELEASE_ASSERT(js::AllowGCBarriers(CycleCollectedJSContext::Get()->Context()),
-                     "sending image notification to JS observer during painting. See bug 1311841");
-
   if (aType == imgINotificationObserver::SIZE_AVAILABLE) {
     return mInner->SizeAvailable(aRequest);
   }