bug 1072150, backout the release assertion which changed the ancient requirements of nsContentUtils::IsCallerChrome(), a=backout

dom/base/nsContentUtils.cpp

@@ -2710,7 +2710,7 @@ nsContentUtils::SubjectPrincipal()
   MOZ_ASSERT(NS_IsMainThread());
   JSContext* cx = GetCurrentJSContext();
   if (!cx) {
-    MOZ_CRASH("Accessing the Subject Principal without an AutoJSAPI on the stack is forbidden");
+    return GetSystemPrincipal();
   }
 
   JSCompartment *compartment = js::GetContextCompartment(cx);