Read got_revision_mapping from built in spec

Sometimes the spec passed in via parameters also have got_revision_mapping that
we want to use.

BUG=590788

Review URL: https://codereview.chromium.org/1748143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299015 0039d316-1c4b-4281-b951-d872f2087c98

recipe_modules/bot_update/api.py

@@ -131,9 +131,7 @@ class BotUpdateApi(recipe_api.RecipeApi):
     else:
       email_file = key_file = None
 
-    rev_map = {}
-    if self.m.gclient.c:
-      rev_map = self.m.gclient.c.got_revision_mapping.as_jsonish()
+    rev_map = cfg.got_revision_mapping.as_jsonish()
 
     flags = [
         # 1. Do we want to run? (master/builder/slave).