Make setting length on arrays faster.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 4355fe9e1fb6b72db3400b1b227ee32d0301b0e0..455a84c8d0747a098841270b4f7db3f4bfb3bf51 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2773,6 +2773,13 @@ bool JSObject::HasIndexedInterceptor() {
 }
 
 
+bool JSObject::AllowsSetElementsLength() {
+  bool result = elements()->IsFixedArray();
+  ASSERT(result == (!HasPixelElements() && !HasExternalArrayElements()));
+  return result;
+}
+
+
 StringDictionary* JSObject::property_dictionary() {
   ASSERT(!HasFastProperties());
   return StringDictionary::cast(properties());