depot_tools: expose path to gn.py

BUG=chromium:602906

Review URL: https://codereview.chromium.org/1902043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@300016 0039d316-1c4b-4281-b951-d872f2087c98

recipe_modules/depot_tools/api.py

@@ -9,6 +9,10 @@ class DepotToolsApi(recipe_api.RecipeApi):
   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')