Be explicit about target type in platform_util::OpenItem()

chrome/chrome_tests_unit.gypi

 # Copyright 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': {
     'chrome_unit_tests_sources': [
       '../components/autofill/content/renderer/test_password_autofill_agent.cc',
       '../components/autofill/content/renderer/test_password_autofill_agent.h',
       '../components/autofill/content/renderer/test_password_generation_agent.cc',
       '../components/autofill/content/renderer/test_password_generation_agent.h',
@@ skipping 1415 matching lines @@
       'browser/media_galleries/media_galleries_dialog_controller_test_util.cc',
       'browser/media_galleries/media_galleries_dialog_controller_test_util.h',
       'browser/media_galleries/media_galleries_permission_controller_unittest.cc',
       'browser/media_galleries/media_galleries_permissions_unittest.cc',
       'browser/media_galleries/media_galleries_preferences_unittest.cc',
       'browser/media_galleries/media_galleries_scan_result_controller_unittest.cc',
       'browser/media_galleries/media_scan_manager_unittest.cc',
       'browser/media_galleries/win/mtp_device_delegate_impl_win_unittest.cc',
       'browser/media_galleries/win/mtp_device_object_enumerator_unittest.cc',
       'browser/net/firefox_proxy_settings_unittest.cc',
+      'browser/platform_util_unittest.cc',
       'browser/power/process_power_collector_unittest.cc',
       'browser/process_singleton_posix_unittest.cc',
       'browser/profile_resetter/profile_resetter_unittest.cc',
       'browser/renderer_context_menu/render_view_context_menu_unittest.cc',
       'browser/search/instant_service_unittest.cc',
       'browser/search/search_unittest.cc',
       # GCMDriverAndroid is not yet implemented.
       'browser/services/gcm/gcm_profile_service_unittest.cc',
       'browser/sessions/persistent_tab_restore_service_unittest.cc',
       'browser/sync/sessions/sessions_sync_manager_unittest.cc',
@@ skipping 1301 matching lines @@
               'dependencies': [
                 '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
               ],
             }],
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }