cbuildbot: Fix mock-tree-status flag.

This flag has been broken since the migration to cbuildbot/tree_status.py
module. FixIt.

BUG=None
TEST=None

Change-Id: I2ebeea15b9799362fb58a53ef021f0ccd332fc98
Reviewed-on: https://chromium-review.googlesource.com/237111
Trybot-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 25ce51b..11a0b98 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -33,6 +33,7 @@
 from chromite.cbuildbot import repository
 from chromite.cbuildbot import results_lib
 from chromite.cbuildbot import tee
+from chromite.cbuildbot import tree_status
 from chromite.cbuildbot import trybot_patch_pool
 from chromite.cbuildbot.stages import afdo_stages
 from chromite.cbuildbot.stages import artifact_stages
@@ -1862,7 +1863,7 @@
           build_config, options)
 
     if options.mock_tree_status is not None:
-      stack.Add(mock.patch.object, timeout_util, '_GetStatus',
+      stack.Add(mock.patch.object, tree_status, '_GetStatus',
                 return_value=options.mock_tree_status)
 
     if options.mock_slave_status is not None: