[Mac]Implement ViewID support.

chrome/browser/cocoa/button_with_viewid.h

+// 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.
+
+#import <Cocoa/Cocoa.h>
+#include "chrome/browser/view_ids.h"
+
+// Class for buttons that have ViewIDs.
+@interface ButtonWithViewID : NSButton {
+ @private
+  // An optional command associated to this button, such as IDC_STOP, etc.
+  int command_;
+}
+
+@property(assign, nonatomic) int command;
+
+// We use NSView's tag property for ViewID support. This method just stores
+// the ViewID to the tag property. So do not use the tag property for other
+// purpose.
+- (void)setViewID:(ViewID)viewID;
+
+@end  // @interface ButtonWithViewID