Add an identity (id) to system drag & drop.

base/base_drop_target.cc

Index: base/base_drop_target.cc
diff --git a/base/base_drop_target.cc b/base/base_drop_target.cc
index 9809255047835b58ab0baee134793fd17bd33020..1dda3963fdc57f152f1deb92444a2df745481339 100644
--- a/base/base_drop_target.cc
+++ b/base/base_drop_target.cc
@@ -11,6 +11,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 IDropTargetHelper* BaseDropTarget::cached_drop_target_helper_ = NULL;
+int32 BaseDropTarget::drag_identity_ = 0;
 
 BaseDropTarget::BaseDropTarget(HWND hwnd)
     : hwnd_(hwnd),
@@ -53,6 +54,11 @@ HRESULT BaseDropTarget::DragEnter(IDataObject* data_object,
     *effect = DROPEFFECT_NONE;
     return S_OK;
   }
+
+  // Update the drag identity, skipping 0.
+  if (++drag_identity_ == 0)
+    ++drag_identity_;
+
   current_data_object_ = data_object;
   POINT screen_pt = { cursor_position.x, cursor_position.y };
   *effect = OnDragEnter(current_data_object_, key_state, screen_pt, *effect);