[Mac] Create HyperlinkButtonCell and make the "Get themes" button an instance

chrome/browser/cocoa/hyperlink_button_cell.h

+// Copyright (c) 2009 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 "base/scoped_nsobject.h"
+
+// A HyperlinkButtonCell is used to create an NSButton that looks and acts
+// like a hyperlink. The default styling is to look like blue, underlined text
+// and to have the pointingHand cursor on mouse over.
+@interface HyperlinkButtonCell : NSButtonCell {
+  scoped_nsobject<NSColor> textColor_;
+}
+@property (retain) NSColor* textColor;
+
+@end