[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/site_utils/suite_scheduler/driver.py b/site_utils/suite_scheduler/driver.py
index 68eb282..7651d81 100644
--- a/site_utils/suite_scheduler/driver.py
+++ b/site_utils/suite_scheduler/driver.py
@@ -11,9 +11,14 @@
 import task, timed_event
 
 import common
+from autotest_lib.client.common_lib import utils
 from autotest_lib.server import utils
 
-from chromite.lib import metrics
+try:
+    from chromite.lib import metrics
+except ImportError:
+    metrics = utils.metrics_mock
+
 
 POOL_SIZE = 32