[Mac]Implement ViewID support.

chrome/browser/cocoa/download_shelf_view.mm

Index: chrome/browser/cocoa/download_shelf_view.mm
diff --git a/chrome/browser/cocoa/download_shelf_view.mm b/chrome/browser/cocoa/download_shelf_view.mm
index 567b77d7ed30f6b89ab0123389696e4bed8113cb..3cfc507a298cb4af3408d6013cc1ec662afc6085 100644
--- a/chrome/browser/cocoa/download_shelf_view.mm
+++ b/chrome/browser/cocoa/download_shelf_view.mm
@@ -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.
 
@@ -7,6 +7,7 @@
 #include "base/scoped_nsobject.h"
 #include "chrome/browser/browser_theme_provider.h"
 #import "chrome/browser/cocoa/themed_window.h"
+#import "chrome/browser/cocoa/view_id_util.h"
 #include "grit/theme_resources.h"
 
 @implementation DownloadShelfView
@@ -63,4 +64,9 @@
   return NO;
 }
 
+// Tag is used solely for ViewID. Override to prevent changing of it.
+- (NSInteger)tag {
+  return view_id_util::ViewIDToTag(VIEW_ID_DOWNLOAD_SHELF);
+}
+
 @end