[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/server/frontend.py b/server/frontend.py
index 2e2eccc..7820a5f 100644
--- a/server/frontend.py
+++ b/server/frontend.py
@@ -20,7 +20,6 @@
 import re
 
 import common
-from chromite.lib import metrics
 
 from autotest_lib.frontend.afe import rpc_client_lib
 from autotest_lib.client.common_lib import control_data
@@ -29,6 +28,10 @@
 from autotest_lib.client.common_lib import utils
 from autotest_lib.tko import db
 
+try:
+    from chromite.lib import metrics
+except ImportError:
+    metrics = utils.metrics_mock
 
 try:
     from autotest_lib.server.site_common import site_utils as server_utils