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

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

Index: third_party/WebKit/Source/core/clipboard/DataTransfer.cpp
diff --git a/third_party/WebKit/Source/core/clipboard/DataTransfer.cpp b/third_party/WebKit/Source/core/clipboard/DataTransfer.cpp
index 2738516710bd335ed2df5c41eca8df85e30a864e..4a9232872ad78139e64744c2c21025501f09045c 100644
--- a/third_party/WebKit/Source/core/clipboard/DataTransfer.cpp
+++ b/third_party/WebKit/Source/core/clipboard/DataTransfer.cpp
@@ -192,12 +192,19 @@ void DataTransfer::setData(const String& type, const String& data) {
   data_object_->SetData(NormalizeType(type), data);
 }
 
-// extensions beyond IE's API
-Vector<String> DataTransfer::types() const {
-  Vector<String> types;
+bool DataTransfer::hasDataStoreItemListChanged() const {
+  return data_store_item_list_changed_ || !CanReadTypes();
+}
+
+void DataTransfer::OnItemListChanged() {
+  data_store_item_list_changed_ = true;
+}
+
+Vector<String> DataTransfer::types() {
   if (!CanReadTypes())
-    return types;
+    return Vector<String>();
 
+  data_store_item_list_changed_ = false;
   return data_object_->Types();
 }
 
@@ -429,7 +436,10 @@ DataTransfer::DataTransfer(DataTransferType type,
       drop_effect_("uninitialized"),
       effect_allowed_("uninitialized"),
       transfer_type_(type),
-      data_object_(data_object) {}
+      data_object_(data_object),
+      data_store_item_list_changed_(true) {
+  data_object_->AddObserver(this);
+}
 
 void DataTransfer::setDragImage(ImageResourceContent* image,
                                 Node* node,
@@ -461,7 +471,7 @@ bool DataTransfer::HasStringOfType(const String& type) const {
   if (!CanReadTypes())
     return false;
 
-  return types().Contains(type);
+  return data_object_->Types().Contains(type);
 }
 
 DragOperation ConvertDropZoneOperationToDragOperation(