[Sync] Do not deadlock when joining sync thread with a pending HistoryModelWorker task.

components/history/core/browser/BUILD.gn

 # Copyright 2014 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.
 
 static_library("browser") {
   sources = [
     "delete_directive_handler.cc",
     "delete_directive_handler.h",
     "download_constants.h",
     "download_database.cc",
@@ skipping 172 matching lines @@
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "download_slice_info_unittest.cc",
     "expire_history_backend_unittest.cc",
     "history_backend_db_unittest.cc",
     "history_backend_unittest.cc",
     "history_database_unittest.cc",
+    "history_model_worker_unittest.cc",
     "history_querying_unittest.cc",
     "history_service_unittest.cc",
     "history_types_unittest.cc",
     "thumbnail_database_unittest.cc",
     "top_sites_cache_unittest.cc",
     "top_sites_database_unittest.cc",
     "top_sites_impl_unittest.cc",
     "typed_url_sync_metadata_database_unittest.cc",
     "typed_url_syncable_service_unittest.cc",
     "url_database_unittest.cc",
@@ skipping 19 matching lines @@
     "//sql:test_support",
     "//testing/gtest",
     "//ui/gfx",
     "//ui/gfx:test_support",
     "//url",
   ]
   if (is_android) {
     sources += [ "android/android_history_types_unittest.cc" ]
   }
 }