DataTransfer: Make |types| be a FrozenArray<DOMString>.

third_party/WebKit/Source/core/clipboard/DataTransfer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 150 matching lines @@
   }
 
   if (CanWriteData())
     effect_allowed_ = effect;
 }
 
 void DataTransfer::clearData(const String& type) {
   if (!CanWriteData())
     return;
 
+  const size_t old_length = data_object_->length();
+
   if (type.IsNull())
     data_object_->ClearAll();
   else
     data_object_->ClearData(NormalizeType(type));
+
+  if (old_length != data_object_->length())
+    data_store_item_list_changed_ = true;
 }
 
 String DataTransfer::getData(const String& type) const {
   if (!CanReadData())
     return String();
 
   bool convert_to_url = false;
   String data = data_object_->GetData(NormalizeType(type, &convert_to_url));
   if (!convert_to_url)
     return data;
   return ConvertURIListToURL(data);
 }
 
 void DataTransfer::setData(const String& type, const String& data) {
   if (!CanWriteData())
     return;
 
   data_object_->SetData(NormalizeType(type), data);
+  data_store_item_list_changed_ = true;
 }
 
-// extensions beyond IE's API
+bool DataTransfer::hasDataStoreItemListChanged() {
+  if (!data_store_item_list_changed_)
+    return false;
+  data_store_item_list_changed_ = false;

[jsbell, 2017/05/12 17:07:35]

Hrm... most places where we use [CachedAttribute] the 
hasXXXChanged()/isXXXDirty() bool method is const and the flag/state is reset in
the xxx() accessor.

NavigatorLanguage is the only one like this. History and the 5 in the modules
are const methods. 

What do you think about changing this? i.e. make this method const, and move the
'... = false' line into types()

+  return true;
+}
+
 Vector<String> DataTransfer::types() const {
   Vector<String> types;
   if (!CanReadTypes())
     return types;
 
   return data_object_->Types();
 }
 
 FileList* DataTransfer::files() const {
   FileList* files = FileList::Create();
@@ skipping 216 matching lines @@
   return data_object_;
 }
 
 DataTransfer::DataTransfer(DataTransferType type,
                            DataTransferAccessPolicy policy,
                            DataObject* data_object)
     : policy_(policy),
       drop_effect_("uninitialized"),
       effect_allowed_("uninitialized"),
       transfer_type_(type),
-      data_object_(data_object) {}
+      data_object_(data_object),
+      data_store_item_list_changed_(true) {}
 
 void DataTransfer::setDragImage(ImageResourceContent* image,
                                 Node* node,
                                 const IntPoint& loc) {
   if (!CanSetDragImage())
     return;
 
   drag_image_ = image;
   drag_loc_ = loc;
   drag_image_element_ = node;
@@ skipping 45 matching lines @@
   }
 }
 
 DEFINE_TRACE(DataTransfer) {
   visitor->Trace(data_object_);
   visitor->Trace(drag_image_);
   visitor->Trace(drag_image_element_);
 }
 
 }  // namespace blink