Bug 1398665 - Handle the case when gNeckoChild is null properly in RequestContext::DOMContentLoaded. r=hurley

In child process, during shutdown, it's possible that IsNeckoChild() is true but gNeckoChild is null. When this case happens, we have to early return in RequestContext::DOMContentLoaded() to avoid the asseration failure (MOZ_ASSERT(!IsNeckoChild())) in RequestContext::ScheduleUnblock().

netwerk/base/RequestContextService.cpp

@@ -125,9 +125,11 @@ RequestContext::DOMContentLoaded()
 
   LOG(("RequestContext::DOMContentLoaded %p", this));
 
-  if (IsNeckoChild() && gNeckoChild) {
+  if (IsNeckoChild()) {
     // Tailing is not supported on the child process
+    if (gNeckoChild) {
       gNeckoChild->SendRequestContextAfterDOMContentLoaded(mID);
+    }
     return NS_OK;
   }