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

third_party/WebKit/Source/core/clipboard/DataObject.h

 /*
  * Copyright (c) 2008, 2009, Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 37 matching lines @@
 class WebDragData;
 
 // A data object for holding data that would be in a clipboard or moved
 // during a drag-n-drop operation. This is the data that WebCore is aware
 // of and is not specific to a platform.
 class CORE_EXPORT DataObject : public GarbageCollectedFinalized<DataObject>,
                                public Supplementable<DataObject> {
   USING_GARBAGE_COLLECTED_MIXIN(DataObject);
 
  public:
+  struct Observer : public GarbageCollectedMixin {
+    virtual void OnItemListChanged() {}

[jsbell, 2017/05/15 20:49:28]

Make this pure virtual? i.e. replace `{}` with `= 0;` ?

[Raphael Kubo da Costa (rakuco), 2017/05/16 08:51:51]

Done.

+  };
+
   static DataObject* CreateFromPasteboard(PasteMode);
   static DataObject* CreateFromString(const String&);
   static DataObject* Create();
   static DataObject* Create(WebDragData);
 
   virtual ~DataObject();
 
   // DataTransferItemList support.
   size_t length() const;
   DataObjectItem* Item(unsigned long index);
@@ skipping 36 matching lines @@
 
   // Used to handle files (images) being dragged out.
   void AddSharedBuffer(PassRefPtr<SharedBuffer>,
                        const KURL&,
                        const String& filename_extension,
                        const AtomicString& content_disposition);
 
   int GetModifiers() const { return modifiers_; }
   void SetModifiers(int modifiers) { modifiers_ = modifiers; }
 
+  // Adds an observer (and retains a reference to it) that is notified
+  // whenever the underlying item_list_ changes.
+  void AddObserver(Observer*);
+
   DECLARE_TRACE();
 
   WebDragData ToWebDragData();
 
  private:
   DataObject();
 
   DataObjectItem* FindStringItem(const String& type) const;
   bool InternalAddStringItem(DataObjectItem*);
   void InternalAddFileItem(DataObjectItem*);
 
+  void NotifyItemListChanged() const;
+
   HeapVector<Member<DataObjectItem>> item_list_;
+  HeapHashSet<Member<Observer>> observers_;
 
   // State of Shift/Ctrl/Alt/Meta keys and Left/Right/Middle mouse buttons
   int modifiers_;
   String filesystem_id_;
 };
 
 }  // namespace blink
 
 #endif