Bug 1541600 part 3. Stop using AutoJSContext in GetStateObject. r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D26225

Author: bzbarsky

dom/base/Document.cpp

@@ -9168,15 +9168,13 @@ nsresult Document::GetStateObject(nsIVariant** aState) {
   // current state object.
 
   if (!mStateObjectCached && mStateObjectContainer) {
-    AutoJSContext cx;
-    nsIGlobalObject* sgo = GetScopeObject();
-    NS_ENSURE_TRUE(sgo, NS_ERROR_UNEXPECTED);
-    JS::Rooted<JSObject*> global(cx, sgo->GetGlobalJSObject());
-    NS_ENSURE_TRUE(global, NS_ERROR_UNEXPECTED);
-    JSAutoRealm ar(cx, global);
-
+    AutoJSAPI jsapi;
+    // Init with null is "OK" in the sense that it will just fail.
+    if (!jsapi.Init(GetScopeObject())) {
+      return NS_ERROR_UNEXPECTED;
+    }
     mStateObjectContainer->DeserializeToVariant(
-        cx, getter_AddRefs(mStateObjectCached));
+        jsapi.cx(), getter_AddRefs(mStateObjectCached));
   }
 
   NS_IF_ADDREF(*aState = mStateObjectCached);