[Mac]Implement ViewID support.

chrome/browser/cocoa/base_view.h

Index: chrome/browser/cocoa/base_view.h
diff --git a/chrome/browser/cocoa/base_view.h b/chrome/browser/cocoa/base_view.h
index d59a2c2ffbc5f225f7a750ec2405171059748098..48b36b729a8c5fb0bf2b69c54d30b62fe12b97fe 100644
--- a/chrome/browser/cocoa/base_view.h
+++ b/chrome/browser/cocoa/base_view.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -8,20 +8,29 @@
 #import <Cocoa/Cocoa.h>
 
 #include "base/scoped_nsobject.h"
+#include "chrome/browser/view_ids.h"
 #include "gfx/rect.h"
 
 // A view that provides common functionality that many views will need:
 // - Automatic registration for mouse-moved events.
 // - Funneling of mouse and key events to two methods
 // - Coordinate conversion utilities
+// - ViewID support.
+//
+// We use NSView's tag property for ViewID support, BaseView class overrides
+// the tag method to return its ViewID value. So do not use the tag property
+// for other purpose in subclasses.
 
 @interface BaseView : NSView {
  @private
   NSTrackingArea *trackingArea_;
   BOOL dragging_;
   scoped_nsobject<NSEvent> pendingExitEvent_;
+  ViewID viewID_;
 }
 
+@property(assign, nonatomic) ViewID viewID;
+
 - (id)initWithFrame:(NSRect)frame;
 
 // Override these methods in a subclass.