[Android] Add gn support for multidex.

build/android/main_dex_action.gypi

 # Copyright 2015 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.
 
 # This file is meant to be included into an action to provide a rule that
 # generates a list of classes that must be kept in the main dex file.
 #
 # To use this, create a gyp target with the following form:
 #  {
 #    'action_name': 'some name for the action'
 #    'actions': [
 #      'variables': {
-#        'jar_path': 'path to jar',
-#        'output_path': 'output path'
+#        'jar_paths': ['path to jar', ...],
+#        'output_path': 'output path',
 #      },
 #      'includes': [ 'relative/path/to/main_dex_action.gypi' ],
 #    ],
 #  },
 #
 
 {
   'message': 'Generating main dex classes list for <(jar_path)',
   'variables': {
     'jar_paths%': [],
     'output_path%': '',
     'main_dex_list_script': '<(DEPTH)/build/android/gyp/main_dex_list.py',
     'main_dex_rules_path': '<(DEPTH)/build/android/main_dex_classes.flags',
   },
   'inputs': [
-    '<(jar_path)',
+    '<@(jar_paths)',
     '<(main_dex_list_script)',
     '<(main_dex_rules_path)',
     '<(multidex_configuration_path)',
   ],
   'outputs': [
     '<(output_path)',
   ],
   'action': [
     'python', '<(main_dex_list_script)',
     '--main-dex-list-path', '<(output_path)',
     '--android-sdk-tools', '<(android_sdk_tools)',
     '--main-dex-rules-path', '<(main_dex_rules_path)',
     '--multidex-configuration-path', '<(multidex_configuration_path)',
     '<@(jar_paths)',
   ]
 }