Use ScopedTaskEnvironment instead of MessageLoopForUI in components tests.

components/bookmarks/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.
 
 import("//build/config/ui.gni")
 
 static_library("browser") {
   sources = [
     "base_bookmark_model_observer.cc",
     "base_bookmark_model_observer.h",
@@ skipping 94 matching lines @@
 
   if (toolkit_views) {
     sources += [ "bookmark_node_data_unittest.cc" ]
   }
 
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
   deps = [
     ":browser",
     ":unit_tests_bundle_data",
+    "//base/test:test_support",
     "//components/bookmarks/common",
     "//components/bookmarks/test",
     "//components/favicon_base",
     "//components/pref_registry",
     "//components/prefs",
     "//components/prefs:test_support",
     "//testing/gtest",
     "//ui/base",
     "//url",
   ]
 }