Re-land alexbost's experimental Offscreen Tabs API work.

chrome/chrome_tests.gypi

 # 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.
 {
   'variables' : {
     'pyautolib_sources': [
       'app/chrome_command_ids.h',
       'app/chrome_dll_resource.h',
       'common/automation_constants.h',
       'common/pref_names.cc',
@@ skipping 2608 matching lines @@
         'browser/extensions/extension_install_ui_browsertest.cc',
         'browser/extensions/extension_javascript_url_apitest.cc',
         'browser/extensions/extension_local_filesystem_apitest.cc',
         'browser/extensions/extension_management_api_browsertest.cc',
         'browser/extensions/extension_management_apitest.cc',
         'browser/extensions/extension_management_browsertest.cc',
         'browser/extensions/extension_messages_apitest.cc',
         'browser/extensions/extension_messages_browsertest.cc',
         'browser/extensions/extension_metrics_apitest.cc',
         'browser/extensions/extension_module_apitest.cc',
+        'browser/extensions/extension_offscreen_tabs_apitest.cc',
         'browser/extensions/extension_omnibox_apitest.cc',
         'browser/extensions/extension_override_apitest.cc',
         'browser/extensions/extension_page_capture_apitest.cc',
         'browser/extensions/extension_preference_apitest.cc',
         'browser/extensions/extension_processes_apitest.cc',
         'browser/extensions/extension_proxy_apitest.cc',
         'browser/extensions/extension_resource_request_policy_apitest.cc',
         'browser/extensions/extension_startup_browsertest.cc',
         'browser/extensions/extension_storage_apitest.cc',
         'browser/extensions/extension_tabs_apitest.cc',
@@ skipping 1967 matching lines @@
               # Use outputs of this action as inputs for the main target build.
               # Seems as a misnomer but makes this happy on Linux (scons).
               'process_outputs_as_sources': 1,
             },
           ],  # 'actions'
         },
       ]
     }],  # 'coverage!=0'
   ],  # 'conditions'
 }