Move recipe engine to third_party/recipe_engine.

scripts/slave/recipe_modules/gatekeeper/api.py

 # 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.
 
-from slave import recipe_api
+from recipe_engine import recipe_api
 
 
 class Gatekeeper(recipe_api.RecipeApi):
   """Module for Gatekeeper NG."""
   def __call__(self, gatekeeper_json, gatekeeper_trees_json):
     config = self.m.json.read(
       'reading %s' % self.m.path.basename(gatekeeper_trees_json),
       gatekeeper_trees_json,
     ).json.output
 
@@ skipping 30 matching lines @@
         args.extend(tree_args['masters'])
 
       try:
         self.m.python(
           'gatekeeper: %s' % str(tree_name),
           self.m.path['build'].join('scripts', 'slave', 'gatekeeper_ng.py'),
           args,
         )
       except self.m.step.StepFailure:
         pass