Enable background mode on Mac by default.

chrome/browser/background_mode_manager.cc

Index: chrome/browser/background_mode_manager.cc
diff --git a/chrome/browser/background_mode_manager.cc b/chrome/browser/background_mode_manager.cc
index c3ffdef1392d29409f93ee73528acdd8c3823824..b72e8c1e6b1e8a1f4a63f99129dd81e6f4682f5e 100644
--- a/chrome/browser/background_mode_manager.cc
+++ b/chrome/browser/background_mode_manager.cc
@@ -368,9 +368,9 @@ bool BackgroundModeManager::IsBackgroundModeEnabled(
   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