Add base::RunLoop and update ui_test_utils to use it to reduce flakiness

base/base.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.
 
 {
   'target_defaults': {
     'variables': {
       'base_target': 0,
     },
     'target_conditions': [
@@ skipping 272 matching lines @@
           'profiler/alternate_timer.cc',
           'profiler/alternate_timer.h',
           'profiler/tracked_time.cc',
           'profiler/tracked_time.h',
           'property_bag.cc',
           'property_bag.h',
           'rand_util.cc',
           'rand_util.h',
           'rand_util_posix.cc',
           'rand_util_win.cc',
+          'run_loop.cc',
+          'run_loop.h',
           'safe_strerror_posix.cc',
           'safe_strerror_posix.h',
           'scoped_native_library.cc',
           'scoped_native_library.h',
           'scoped_temp_dir.cc',
           'scoped_temp_dir.h',
           'sequenced_task_runner.cc',
           'sequenced_task_runner.h',
           'sequenced_task_runner_helpers.h',
           'sha1.h',
@@ skipping 311 matching lines @@
           ['<(chromeos)!=1 or >(nacl_untrusted_build)==1', {
             'sources/': [
               ['exclude', '^chromeos/'],
             ],
           }],
         ],
       }],
     ],
   },
 }