Add missing IWYU message_loop.h includes.

chrome/browser/plugins/plugin_power_saver_browsertest.cc

 // Copyright 2015 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.
 
 #include <stdint.h>
 #include <string>
 #include <utility>
 
 #include "base/command_line.h"
+#include "base/message_loop/message_loop.h"
 #include "base/stl_util.h"
 #include "base/strings/string_piece.h"
 #include "base/strings/stringprintf.h"
 #include "base/test/scoped_feature_list.h"
 #include "base/threading/thread_restrictions.h"
 #include "build/build_config.h"
 #include "chrome/browser/content_settings/host_content_settings_map_factory.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_window.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
@@ skipping 616 matching lines @@
 IN_PROC_BROWSER_TEST_F(PluginPowerSaverPreferHtmlBrowserTest,
                        ThrottlePluginsOnAllowContentSetting) {
   HostContentSettingsMap* content_settings_map =
       HostContentSettingsMapFactory::GetForProfile(browser()->profile());
 
   content_settings_map->SetDefaultContentSetting(CONTENT_SETTINGS_TYPE_PLUGINS,
                                                  CONTENT_SETTING_ALLOW);
   LoadPeripheralPlugin();
   VerifyPluginIsThrottled(GetActiveWebContents(), "plugin");
 }