Reland (3rd try): Lazily initialize MessageLoop for faster thread startup

content/browser/browser_main_loop.cc

 // Copyright (c) 2012 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 "content/browser/browser_main_loop.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/logging.h"
 #include "base/memory/memory_pressure_monitor.h"
 #include "base/message_loop/message_loop.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/pending_task.h"
 #include "base/power_monitor/power_monitor.h"
 #include "base/power_monitor/power_monitor_device_source.h"
 #include "base/process/process_metrics.h"
 #include "base/profiler/scoped_profile.h"
 #include "base/run_loop.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
+#include "base/synchronization/lock.h"

[kinuko, 2015/05/22 11:58:40]

Unnecessary change; will remove this

 #include "base/system_monitor/system_monitor.h"
 #include "base/thread_task_runner_handle.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/timer/hi_res_timer_manager.h"
 #include "base/trace_event/memory_dump_manager.h"
 #include "base/trace_event/trace_event.h"
 #include "content/browser/browser_thread_impl.h"
 #include "content/browser/device_sensors/device_inertial_sensor_service.h"
 #include "content/browser/dom_storage/dom_storage_area.h"
 #include "content/browser/download/save_file_manager.h"
@@ skipping 1297 matching lines @@
 
 void BrowserMainLoop::EndStartupTracing() {
   is_tracing_startup_ = false;
   TracingController::GetInstance()->DisableRecording(
       TracingController::CreateFileSink(
           startup_trace_file_,
           base::Bind(OnStoppedStartupTracing, startup_trace_file_)));
 }
 
 }  // namespace content