Enable background mode on Mac by default.

chrome/browser/background_mode_manager.cc

 // Copyright (c) 2010 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 "app/l10n_util.h"
 #include "app/resource_bundle.h"
 #include "base/base_paths.h"
 #include "base/command_line.h"
 #include "base/logging.h"
 #include "base/utf_string_conversions.h"
@@ skipping 350 matching lines @@
 
 // static
 bool BackgroundModeManager::IsBackgroundModeEnabled(
     const CommandLine* command_line) {
 
   // Background mode is disabled if the appropriate flag is passed, or if
   // extensions are disabled.
   bool background_mode_enabled =
       !command_line->HasSwitch(switches::kDisableBackgroundMode) &&
       !command_line->HasSwitch(switches::kDisableExtensions);
-#if !defined(OS_WIN)
-  // BackgroundMode is enabled by default on windows. On other platforms, it
-  // is enabled via about:flags.
+#if !(defined(OS_WIN) || defined(OS_MACOSX))
+  // BackgroundMode is enabled by default on windows and mac. On other
+  // platforms, it is enabled via about:flags.
   background_mode_enabled = background_mode_enabled &&
       command_line->HasSwitch(switches::kEnableBackgroundMode);
 #endif
 
   return background_mode_enabled;
 }