goma: use GOMA_SERVICE_ACCOUNT_JSON_FILE (reland #1)

scripts/slave/recipe_modules/goma/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.
 
 from recipe_engine import recipe_api
 
 class GomaApi(recipe_api.RecipeApi):
   """GomaApi contains helper functions for using goma."""
 
+  @property
+  def service_account_json_path(self):
+    if self.m.platform.is_win:
+      return 'C:\\creds\\service_accounts\\service-account-goma-client.json'
+    return '/creds/service_accounts/service-account-goma-client.json'
+
   def update_goma_canary(self):
     """Returns a step for updating goma canary."""
     # deprecated? switch to use ensure_goma with canary=True.
     # for git checkout, should use @refs/heads/master to use head.
     head = 'refs/heads/master'
     # TODO(phajdan.jr): Remove path['build'] usage, http://crbug.com/437264 .
     self.m.gclient('update goma canary',
                    ['sync', '--verbose', '--force',
                     '--revision', 'build/goma@%s' % head],
                    cwd=self.m.path['build'])
 
   def ensure_goma(self):
     with self.m.step.nest('ensure_goma'):
       try:
-        if self.m.platform.is_win:
-          creds = ('C:\\creds\\service_accounts\\'
-                   'service-account-goma-client.json')
-        else:
-          creds = '/creds/service_accounts/service-account-goma-client.json'
-        self.m.cipd.set_service_account_credentials(creds)
+        self.m.cipd.set_service_account_credentials(
+            self.service_account_json_path)
 
         self.m.cipd.install_client()
         goma_package = ('infra_internal/goma/client/%s' %
             self.m.cipd.platform_suffix())
         goma_dir = self.m.path['cache'].join('cipd', 'goma')
         self.m.cipd.ensure(goma_dir, {goma_package: 'release'})
         return goma_dir
       except self.m.step.StepFailure:  # pragma: no cover
         # TODO(phajdan.jr): make failures fatal after experiment.
         return None