[autotest] Use the metrics_mock object in case chromite is not set up.

BUG=chromium:688166
TEST=unittest

Change-Id: Ic0077cb2dba75a8d820f229060f3f70f507850a1
Reviewed-on: https://chromium-review.googlesource.com/438754
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Dan Shi <dshi@google.com>
diff --git a/scheduler/postjob_task.py b/scheduler/postjob_task.py
index 9310c57..67733f5 100644
--- a/scheduler/postjob_task.py
+++ b/scheduler/postjob_task.py
@@ -10,13 +10,18 @@
 
 import os
 
+from autotest_lib.client.common_lib import utils
 from autotest_lib.frontend.afe import models, model_attributes
 from autotest_lib.scheduler import agent_task, drones, drone_manager
 from autotest_lib.scheduler import email_manager, pidfile_monitor
 from autotest_lib.scheduler import scheduler_config
 from autotest_lib.server import autoserv_utils
 
-from chromite.lib import metrics
+try:
+    from chromite.lib import metrics
+except ImportError:
+    metrics = utils.metrics_mock
+
 
 _parser_path = os.path.join(drones.AUTOTEST_INSTALL_DIR, 'tko', 'parse')