[Sync] Make syncer commands avoid posting tasks on threads with no work to do

chrome/chrome_tests.gypi

 # Copyright (c) 2011 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 3217 matching lines @@
     {
       'target_name': 'sync_unit_tests',
       'type': 'executable',
       'sources': [
         '<(protoc_out_dir)/chrome/browser/sync/protocol/test.pb.cc',
         'app/breakpad_mac_stubs.mm',
         'browser/sync/api/sync_change_unittest.cc',
         'browser/sync/api/sync_error_unittest.cc',
         'browser/sync/engine/apply_updates_command_unittest.cc',
         'browser/sync/engine/build_commit_command_unittest.cc',
+        'browser/sync/engine/build_and_process_conflict_sets_command_unittest.cc',
         'browser/sync/engine/clear_data_command_unittest.cc',
         'browser/sync/engine/cleanup_disabled_types_command_unittest.cc',
         'browser/sync/engine/download_updates_command_unittest.cc',
+        'browser/sync/engine/model_changing_syncer_command_unittest.cc',
         'browser/sync/engine/model_safe_worker_unittest.cc',
         'browser/sync/engine/nigori_util_unittest.cc',
         'browser/sync/engine/process_commit_response_command_unittest.cc',
+        'browser/sync/engine/process_updates_command_unittest.cc',
+        'browser/sync/engine/resolve_conflicts_command_unittest.cc',
         'browser/sync/engine/syncer_proto_util_unittest.cc',
         'browser/sync/engine/sync_scheduler_unittest.cc',
         'browser/sync/engine/sync_scheduler_whitebox_unittest.cc',
         'browser/sync/engine/syncer_unittest.cc',
         'browser/sync/engine/syncproto_unittest.cc',
         'browser/sync/engine/verify_updates_command_unittest.cc',
         'browser/sync/internal_api/change_record_unittest.cc',
         'browser/sync/internal_api/debug_info_event_listener_unittest.cc',
         'browser/sync/internal_api/syncapi_mock.h',
         'browser/sync/internal_api/syncapi_server_connection_manager_unittest.cc',
@@ skipping 1047 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'
 }