[Mac]Implement ViewID support.

chrome/browser/cocoa/base_view.mm

Index: chrome/browser/cocoa/base_view.mm
diff --git a/chrome/browser/cocoa/base_view.mm b/chrome/browser/cocoa/base_view.mm
index cee89511abfc416f7f6b714a93d8021ae6a0769a..60e4295645b5f6a25ae03bfc73a4e2035ad5743c 100644
--- a/chrome/browser/cocoa/base_view.mm
+++ b/chrome/browser/cocoa/base_view.mm
@@ -1,11 +1,14 @@
-// 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.
 
-#include "chrome/browser/cocoa/base_view.h"
+#import "chrome/browser/cocoa/base_view.h"
+#import "chrome/browser/cocoa/view_id_util.h"
 
 @implementation BaseView
 
+@synthesize viewID = viewID_;
+
 - (id)initWithFrame:(NSRect)frame {
   self = [super initWithFrame:frame];
   if (self) {
@@ -140,4 +143,10 @@
   return new_rect;
 }
 
+// We use NSView's tag prooperty for ViewID support. Subclasses should not
+// override it for other purpose.
+- (NSInteger)tag {
+  return view_id_util::ViewIDToTag(viewID_);
+}
+
 @end