[cocoa] increase tab strip contrast in "Increase Contrast" mode

ui/base/default_theme_provider_mac.mm

 // Copyright (c) 2013 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 "ui/base/default_theme_provider.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "ui/base/resource/resource_bundle.h"
 
@@ skipping 26 matching lines @@
 }
 
 NSColor* DefaultThemeProvider::GetNSColorTint(int id) const {
   return [NSColor redColor];
 }
 
 NSGradient* DefaultThemeProvider::GetNSGradient(int id) const {
   return nil;
 }
 
+bool DefaultThemeProvider::ShouldIncreaseContrast() const {
+  return false;
+}
+
 }  // namespace ui