Clean up d8 ArrayBuffer implementation and fix bug in readbuffer:

src/d8.h

Index: src/d8.h
diff --git a/src/d8.h b/src/d8.h
index b315086fcfdb762e86176832744c8c999c71e06b..2789c6db3ec1be93cb4aa50df79212cb7ce29280 100644
--- a/src/d8.h
+++ b/src/d8.h
@@ -383,9 +383,11 @@ class Shell : public i::AllStatic {
   static void RunShell();
   static bool SetOptions(int argc, char* argv[]);
   static Handle<ObjectTemplate> CreateGlobalTemplate();
+  static Handle<Value> CreateExternalArrayBuffer(int32_t size);
+  static Handle<Value> CreateExternalArrayBuffer(const Arguments& args);
   static Handle<Value> CreateExternalArray(const Arguments& args,
                                            ExternalArrayType type,
-                                           size_t element_size);
+                                           int32_t element_size);
   static void ExternalArrayWeakCallback(Persistent<Value> object, void* data);
 };