Enter context if need be; JavaAdapters may have callbacks on threads unassociated

with contexts.

js/rhino/src/org/mozilla/javascript/JavaAdapter.java

@@ -70,9 +70,16 @@ public class JavaAdapter extends ScriptableObject {
     }
 
     public static Scriptable setAdapterProto(Scriptable obj, Object adapter) {
-        Scriptable res = ScriptRuntime.toObject(ScriptableObject.getTopLevelScope(obj),adapter);
+        // We could be called from a thread not associated with a Context
+        Context cx = Context.enter();
+        try {
+            Scriptable topLevel = ScriptableObject.getTopLevelScope(obj);
+            Scriptable res = ScriptRuntime.toObject(topLevel, adapter);
             res.setPrototype(obj);
             return res;
+        } finally {
+            cx.exit();
+        }
     }
 
     public static Object getAdapterSelf(Class adapterClass, Object adapter)