Array.prototype.unshift builtin improvements

src/elements.h

Index: src/elements.h
diff --git a/src/elements.h b/src/elements.h
index 16be8ecde3564941735e5e093ad920ec4d9e856e..d188cf8c7084fe67e446aa6aeb9932db270ca57f 100644
--- a/src/elements.h
+++ b/src/elements.h
@@ -125,6 +125,10 @@ class ElementsAccessor {
                    Handle<Object> value, PropertyAttributes attributes,
                    uint32_t new_capacity) = 0;
 
+  virtual uint32_t Unshift(Handle<JSArray> receiver,
+                           Handle<FixedArrayBase> backing_store, Arguments args,
+                           uint32_t add_count) = 0;
+
   // TODO(cbruni): Consider passing Arguments* instead of Object** depending on
   // the requirements of future callers.
   virtual uint32_t Push(Handle<JSArray> receiver,