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

chrome/browser/ui/cocoa/download/background_theme.mm

 // Copyright (c) 2012 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/ui/cocoa/download/background_theme.h"
 
 #import "chrome/browser/themes/theme_properties.h"
 
 BackgroundTheme::BackgroundTheme(const ui::ThemeProvider* provider)
     : provider_(provider) {
@@ skipping 78 matching lines @@
     case ThemeProperties::GRADIENT_TOOLBAR_BUTTON:
     case ThemeProperties::GRADIENT_TOOLBAR_BUTTON_INACTIVE:
       return buttonGradient_.get();
     case ThemeProperties::GRADIENT_TOOLBAR_BUTTON_PRESSED:
     case ThemeProperties::GRADIENT_TOOLBAR_BUTTON_PRESSED_INACTIVE:
       return buttonPressedGradient_.get();
     default:
       return provider_->GetNSGradient(id);
   }
 }
+
+bool BackgroundTheme::ShouldIncreaseContrast() const {
+  return false;
+}