ResizingHostObserver is created by the desktop environment together with other stubs.

remoting/remoting.gyp

 # 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': {
     # TODO(dmaclach): can we pick this up some other way? Right now it's
     # duplicated from chrome.gyp
     'chromium_code': 1,
 
@@ skipping 362 matching lines @@
             'host/ipc_audio_capturer.cc',
             'host/ipc_audio_capturer.h',
             'host/ipc_constants.cc',
             'host/ipc_constants.h',
             'host/ipc_desktop_environment.cc',
             'host/ipc_desktop_environment.h',
             'host/ipc_event_executor.cc',
             'host/ipc_event_executor.h',
             'host/ipc_host_event_logger.cc',
             'host/ipc_host_event_logger.h',
+            'host/ipc_session_controller.cc',
+            'host/ipc_session_controller.h',
             'host/ipc_video_frame_capturer.cc',
             'host/ipc_video_frame_capturer.h',
             'host/it2me_host_user_interface.cc',
             'host/it2me_host_user_interface.h',
             'host/json_host_config.cc',
             'host/json_host_config.h',
             'host/linux/audio_pipe_reader.cc',
             'host/linux/audio_pipe_reader.h',
             'host/linux/x11_util.cc',
             'host/linux/x11_util.h',
@@ skipping 27 matching lines @@
             'host/resizing_host_observer.cc',
             'host/resizing_host_observer.h',
             'host/sas_injector.h',
             'host/sas_injector_win.cc',
             'host/server_log_entry.cc',
             'host/server_log_entry.h',
             'host/service_client.cc',
             'host/service_client.h',
             'host/service_urls.cc',
             'host/service_urls.h',
+            'host/session_controller.h',
             'host/session_manager_factory.cc',
             'host/session_manager_factory.h',
             'host/signaling_connector.cc',
             'host/signaling_connector.h',
             'host/ui_strings.cc',
             'host/ui_strings.h',
             'host/url_request_context.cc',
             'host/url_request_context.h',
             'host/usage_stats_consent.h',
             'host/usage_stats_consent_mac.cc',
@@ skipping 2218 matching lines @@
                   '../base/allocator/allocator.gyp:allocator',
                 ],
               },
             ],
           ],
         }],  # end of 'toolkit_uses_gtk == 1'
       ],  # end of 'conditions'
     },  # end of target 'remoting_unittests'
   ],  # end of targets
 }