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

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

Index: third_party/WebKit/Source/core/clipboard/DataObject.h
diff --git a/third_party/WebKit/Source/core/clipboard/DataObject.h b/third_party/WebKit/Source/core/clipboard/DataObject.h
index 2d699c5a2fe10eb9493ec9d01052e094866e7ed7..ac06f46000e599d2d3c06df2907884cc1318d4c8 100644
--- a/third_party/WebKit/Source/core/clipboard/DataObject.h
+++ b/third_party/WebKit/Source/core/clipboard/DataObject.h
@@ -55,6 +55,10 @@ class CORE_EXPORT DataObject : public GarbageCollectedFinalized<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();
@@ -111,6 +115,10 @@ class CORE_EXPORT DataObject : public GarbageCollectedFinalized<DataObject>,
   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();
@@ -122,7 +130,10 @@ class CORE_EXPORT DataObject : public GarbageCollectedFinalized<DataObject>,
   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_;