Refactor UnionOfKeys into ElementsAccessor

src/elements.h

Index: src/elements.h
diff --git a/src/elements.h b/src/elements.h
index b552286f0804798b9e3b35099d92fe56eb00391d..f64244e95856e1de7f548eea8de685ad7bebcbdb 100644
--- a/src/elements.h
+++ b/src/elements.h
@@ -47,6 +47,9 @@ class ElementsAccessor {
                               uint32_t index,
                               JSReceiver::DeleteMode mode) = 0;
 
+  virtual MaybeObject* AddJSArrayKeysToFixedArray(JSArray* other,
+                                                  FixedArray* keys) = 0;
+
   // Returns a shared ElementsAccessor for the specified ElementsKind.
   static ElementsAccessor* ForKind(JSObject::ElementsKind elements_kind) {
     ASSERT(elements_kind < JSObject::kElementsKindCount);