[MERGE #2300 @ThomsonTan] Marshal value when traversing property keys via proxy

Merge pull request #2300 from ThomsonTan:MarshalScriptContextForEnumeratingKeysOnProxy

We missed to marshal property key string when traversing (Object.keys()) an object via a proxy on it. Marshal is necessary because the object underlying the proxy could be from a difference script context.

Author: ThomsonTan

lib/Runtime/Language/JavascriptOperators.cpp

@@ -1176,7 +1176,7 @@ namespace Js
                 {
                     if (propertyDescriptor.IsEnumerable())
                     {
-                        proxyResultToReturn->DirectSetItemAt(index++, element);
+                        proxyResultToReturn->DirectSetItemAt(index++, CrossSite::MarshalVar(scriptContext, element));
                     }
                 }
             }