Do not monitor system power state during unit tests.

chrome/browser/browser_main.cc

 // Copyright (c) 2006-2009 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 "build/build_config.h"
 
 #if defined(OS_WIN)
 #include <windows.h>
 #include <commctrl.h>
 #endif
@@ skipping 247 matching lines @@
 
   // Do platform-specific things (such as finishing initializing Cocoa)
   // prior to instantiating the message loop. This could be turned into a
   // broadcast notification.
   Platform::WillInitializeMainMessageLoop(parsed_command_line);
 
   MessageLoop main_message_loop(MessageLoop::TYPE_UI);
 
   // Initialize the SystemMonitor
   base::SystemMonitor::Start();
+#if defined(OS_WIN)
+  // We want to monitor system power state to adjust our high resolution
+  // timer settings. But it's necessary only on Windows.
+  base::Time::StartSystemMonitorObserver();
+#endif  // defined(OS_WIN)
 
   // Initialize statistical testing infrastructure.
   FieldTrialList field_trial;
 
   std::wstring app_name = chrome::kBrowserAppName;
   std::string thread_name_string = WideToASCII(app_name + L"_BrowserMain");
 
   const char* thread_name = thread_name_string.c_str();
   PlatformThread::SetName(thread_name);
   main_message_loop.set_thread_name(thread_name);
@@ skipping 453 matching lines @@
   if (metrics)
     metrics->Stop();
 
   // browser_shutdown takes care of deleting browser_process, so we need to
   // release it.
   browser_process.release();
   browser_shutdown::Shutdown();
 
   return result_code;
 }