build: use class path bases, not strings

scripts/slave/recipe_modules/emulator/api.py

 # 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.
 
 import contextlib
 
 from recipe_engine import recipe_api
 from recipe_engine.config_types import Path
 
 class EmulatorApi(recipe_api.RecipeApi):
+  def get_config_defaults(self):
+    return {
+        'CHECKOUT_PATH': self.m.path['checkout'],
+    }
 
   def install_emulator_deps(self, api_level, **kwargs):
     args = [
         '--api-level', api_level,
     ]
     return self.m.python('[emulator] installing emulator deps',
                          self.c.install_emulator_deps_path, args, **kwargs)
 
   def wait_for_emulator(self, num, **kwargs):
     args = ['wait', '-n', num]
@@ skipping 30 matching lines @@
 
     try:
       yield
     finally:
       exit_args = ['kill']
       self.m.python('[emulator] killing all emulators', self.c.avd_script_path,
                     exit_args)
       delete_avd_args = ['delete']
       self.m.python('[emulator] deleting all temp avds after running',
                     self.c.avd_script_path, delete_avd_args)