Generalizing architecture for screenshot testing

chrome/chrome_tests.gypi

 # Copyright 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.
 {
   'includes': [
     'js_unittest_vars.gypi',
   ],
   'targets': [
     {
       'target_name': 'interactive_ui_tests',
@@ skipping 289 matching lines @@
             'browser/chromeos/login/lock/screen_locker_tester.h',
             'browser/chromeos/login/login_browsertest.cc',
             'browser/chromeos/login/login_manager_test.cc',
             'browser/chromeos/login/login_manager_test.h',
             'browser/chromeos/login/login_ui_browsertest.cc',
             'browser/chromeos/login/oobe_browsertest.cc',
             'browser/chromeos/login/screenshot_tester.h',
             'browser/chromeos/login/screenshot_tester.cc',
             'browser/chromeos/login/mixin_based_browser_test.h',
             'browser/chromeos/login/mixin_based_browser_test.cc',
+            'browser/chromeos/login/screenshot_testing_mixin.h',
+            'browser/chromeos/login/screenshot_testing_mixin.cc',
             'browser/chromeos/login/users/wallpaper/wallpaper_manager_browsertest.cc',
             'browser/chromeos/login/users/wallpaper/wallpaper_manager_test_utils.cc',
             'browser/chromeos/login/users/wallpaper/wallpaper_manager_test_utils.h',
             'test/data/chromeos/service_login.html',
           ],
           'sources!': [
             '../ui/views/widget/desktop_aura/desktop_window_tree_host_x11_interactive_uitest.cc',
             '../ui/views/widget/desktop_aura/x11_topmost_window_finder_interactive_uitest.cc',
 
             # chromeos does not use cross-platform panels
@@ skipping 2742 matching lines @@
             ['enable_webrtc==1', {
               'dependencies': [
                 '../third_party/libjingle/libjingle.gyp:libjingle_webrtc'
               ]
             }],
           ],
         }]
     }],
   ],  # 'conditions'
 }