components: Introduce AlarmTimer class and use it for GCM heartbeat

components/timer.gypi

 # Copyright 2014 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.
 
 {
   'targets': [
     {
-      'target_name': 'crx_file',
+      # GN version: //components/timer
+      'target_name': 'timer',
       'type': 'static_library',
+      'include_dirs': [
+        '..',
+      ],
       'dependencies': [
         '../base/base.gyp:base',
       ],
-      'include_dirs': [
-        '..',
-      ],
       'sources': [
-        'crx_file/constants.h',
-        'crx_file/crx_file.cc',
-        'crx_file/crx_file.h',
-        'crx_file/id_util.cc',
-        'crx_file/id_util.h',
+        'timer/alarm_timer.cc',
+        'timer/alarm_timer.h',
+        'timer/rtc_alarm.cc',
+        'timer/rtc_alarm.h',
       ],
     },
   ],
-}
+}