goma: use GOMA_SERVICE_ACCOUNT_JSON_FILE (reland #1)

scripts/slave/recipe_modules/goma/api.py

Index: scripts/slave/recipe_modules/goma/api.py
diff --git a/scripts/slave/recipe_modules/goma/api.py b/scripts/slave/recipe_modules/goma/api.py
index 700de45e42ce8b8de84183ada0112429d2b2f767..18f6018b471ded276d9d80c35bef7e273b32567b 100644
--- a/scripts/slave/recipe_modules/goma/api.py
+++ b/scripts/slave/recipe_modules/goma/api.py
@@ -7,6 +7,12 @@ 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.
@@ -21,12 +27,8 @@ class GomaApi(recipe_api.RecipeApi):
   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' %