Enable background mode on Mac by default.

chrome/browser/about_flags.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 "chrome/browser/about_flags.h"
 
 #include <algorithm>
 #include <iterator>
 #include <map>
 #include <set>
@@ skipping 137 matching lines @@
     "xss-auditor",  // FLAGS:RECORD_UMA
     IDS_FLAGS_XSS_AUDITOR_NAME,
     IDS_FLAGS_XSS_AUDITOR_DESCRIPTION,
     kOsAll,
     SINGLE_VALUE_TYPE(switches::kEnableXSSAuditor)
   },
   {
     "background-webapps",  // FLAGS:RECORD_UMA
     IDS_FLAGS_BACKGROUND_WEBAPPS_NAME,
     IDS_FLAGS_BACKGROUND_WEBAPPS_DESCRIPTION,
-    kOsMac | kOsLinux | kOsCrOS,  // Enabled by default on windows
+    kOsLinux,  // Enabled by default on windows and mac, not available on CrOS.
     SINGLE_VALUE_TYPE(switches::kEnableBackgroundMode)
   },
   {
     "conflicting-modules-check",  // FLAGS:RECORD_UMA
     IDS_FLAGS_CONFLICTS_CHECK_NAME,
     IDS_FLAGS_CONFLICTS_CHECK_DESCRIPTION,
     kOsWin,
     SINGLE_VALUE_TYPE(switches::kConflictingModulesCheck)
   },
   {
@@ skipping 521 matching lines @@
 }
 
 const Experiment* GetExperiments(size_t* count) {
   *count = num_experiments;
   return experiments;
 }
 
 }  // namespace testing
 
 }  // namespace about_flags