Add missing IWYU message_loop.h includes.

ios/clean/chrome/browser/ui/tab/tab_coordinator_unittest.mm

 // Copyright 2017 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 "ios/clean/chrome/browser/ui/tab/tab_coordinator.h"
 
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "ios/chrome/browser/browser_state/test_chrome_browser_state.h"
 #import "ios/clean/chrome/browser/ui/tab/tab_container_view_controller.h"
 #import "ios/shared/chrome/browser/ui/browser_list/browser.h"
 #import "ios/shared/chrome/browser/ui/coordinators/browser_coordinator+internal.h"
 #import "ios/shared/chrome/browser/ui/toolbar/toolbar_test_util.h"
 #include "ios/web/public/test/test_web_thread.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/platform_test.h"
 
@@ skipping 70 matching lines @@
             [GetCoordinator().viewController class]);
 }
 
 TEST_F(TabCoordinatorTest, BottomToolbar) {
   [[NSUserDefaults standardUserDefaults] setObject:@"Enabled"
                                             forKey:@"EnableBottomToolbar"];
   [GetCoordinator() start];
   EXPECT_EQ([BottomToolbarTabViewController class],
             [GetCoordinator().viewController class]);
 }