Add APIs to protect critical tasks on iOS.

base/base.gypi

Index: base/base.gypi
diff --git a/base/base.gypi b/base/base.gypi
index 63362f1d84016073c23093403abacaeafea8a6d8..20d3ed21824d05cebc3323bacf85ba328cec924f 100644
--- a/base/base.gypi
+++ b/base/base.gypi
@@ -86,6 +86,9 @@
           'compiler_specific.h',
           'cpu.cc',
           'cpu.h',
+	  'critical_closure.h',
+          'critical_closure_ios.mm',
+          'critical_closure_stub.cc',
           'debug/alias.cc',
           'debug/alias.h',
           'debug/debug_on_start_win.cc',
@@ -151,6 +154,8 @@
           'hi_res_timer_manager_win.cc',
           'hi_res_timer_manager.h',
           'id_map.h',
+          'ios/scoped_critical_action.h',
+          'ios/scoped_critical_action.mm',
           'json/json_file_value_serializer.cc',
           'json/json_file_value_serializer.h',
           'json/json_parser.cc',
@@ -597,6 +602,9 @@
               # needed on iOS (mostly for unit tests).
               ['exclude', '^process_util'],
               ['include', '^process_util_ios\\.mm$'],
+              # Exclude the stub implementation for critical closure on iOS.
+              # 'critical_closure_ios.mm' will be used for iOS.
+              ['exclude', '^critical_closure_stub\\.cc$'],
             ],
           }],
           ['OS != "mac" or >(nacl_untrusted_build)==1', {