Add missing IWYU message_loop.h includes.

extensions/common/one_shot_event_unittest.cc

 // Copyright 2013 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 "extensions/common/one_shot_event.h"
 
 #include "base/bind.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/test/test_simple_task_runner.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace extensions {
 
 namespace {
 
 void Increment(int* i) { ++*i; }
@@ skipping 100 matching lines @@
   EXPECT_EQ(1U, runner->NumPendingTasks());
   EXPECT_EQ(0, i);
   runner->RunPendingTasks();
   // Increment has run.
   EXPECT_EQ(0U, runner->NumPendingTasks());
   EXPECT_EQ(1, i);
 }
 
 }  // namespace
 }  // namespace extensions