Bug 1665862: Don't use AutoSafeJSContext in XMLHttpRequestWorker. r=nika

It crashes if it fails to create the unprivileged junk scope, which is not
great when it's being used by a fallible function.

Differential Revision: https://phabricator.services.mozilla.com/D90753

dom/xhr/XMLHttpRequestWorker.cpp

@@ -898,7 +898,12 @@ Proxy::HandleEvent(Event* aEvent) {
   }
 
   {
-    AutoSafeJSContext cx;
+    AutoJSAPI jsapi;
+    JSObject* junkScope = xpc::UnprivilegedJunkScope(fallible);
+    if (!junkScope || !jsapi.Init(junkScope)) {
+      return NS_ERROR_FAILURE;
+    }
+    JSContext* cx = jsapi.cx();
 
     JS::Rooted<JS::Value> value(cx);
     if (!GetOrCreateDOMReflectorNoWrap(cx, mXHR, &value)) {