Fix setting array length to be ES5 conform.

src/elements.h

Index: src/elements.h
diff --git a/src/elements.h b/src/elements.h
index 851c8c3d97f89d7aab4f0e910ead6c4780b18b5e..ed1ca5e58e93fbbcf76c795838aaec5b10d3f1eb 100644
--- a/src/elements.h
+++ b/src/elements.h
@@ -44,6 +44,11 @@ class ElementsAccessor {
                            JSObject* holder,
                            Object* receiver) = 0;
 
+  // Modifies the length data property as specified for JSArrays and resizes
+  // the underlying backing store accordingly.
+  virtual MaybeObject* SetLength(JSObject* holder,
+                                 Object* new_length) = 0;
+
   virtual MaybeObject* Delete(JSObject* holder,
                               uint32_t key,
                               JSReceiver::DeleteMode mode) = 0;