depot_tools: expose path to gn.py

recipe_modules/depot_tools/api.py

 # Copyright 2016 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.
 
 from recipe_engine import recipe_api
 
 class DepotToolsApi(recipe_api.RecipeApi):
   @property
   def download_from_google_storage_path(self):
     return self.package_repo_resource('download_from_google_storage.py')
 
   @property
+  def gn_py_path(self):
+    return self.package_repo_resource('gn.py')
+
+  @property
   def gsutil_py_path(self):
     return self.package_repo_resource('gsutil.py')
 
   @property
   def ninja_path(self):
     ninja_exe = 'ninja.exe' if self.m.platform.is_win else 'ninja'
     return self.package_repo_resource(ninja_exe)