[autotest] Fix module not found error in gmail_lib

metrics_mock is not located in site_utils. This fixes:

Traceback (most recent call last):
  File "/mnt/host/source/src/third_party/autotest/files/site_utils/test_that.py", line 24, in <module>
    from autotest_lib.site_utils import test_runner_utils
  File "/mnt/host/source/src/third_party/autotest/files/site_utils/test_runner_utils.py", line 28, in <module>
    from autotest_lib.server import autoserv_utils
  File "/mnt/host/source/src/third_party/autotest/files/server/autoserv_utils.py", line 13, in <module>
    from autotest_lib.scheduler import drone_manager
  File "/mnt/host/source/src/third_party/autotest/files/scheduler/drone_manager.py", line 10, in <module>
    from autotest_lib.scheduler import drone_utility
  File "/mnt/host/source/src/third_party/autotest/files/scheduler/drone_utility.py", line 39, in <module>
    from autotest_lib.scheduler import email_manager
  File "/mnt/host/source/src/third_party/autotest/files/scheduler/email_manager.py", line 13, in <module>
    from autotest_lib.site_utils  import gmail_lib
  File "/mnt/host/source/src/third_party/autotest/files/site_utils/gmail_lib.py", line 47, in <module>
    metrics = site_utils.metrics_mock
AttributeError: 'module' object has no attribute 'metrics_mock'

Signed-off-by: Sean Paul <seanpaul@chromium.org>

BUG=None
TEST=test_that doesn't crash when run
Change-Id: I47e164bb31435d103b73e0d9b5a393378720645e
Reviewed-on: https://chromium-review.googlesource.com/552302
Commit-Ready: Sean Paul <seanpaul@chromium.org>
Tested-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
diff --git a/site_utils/gmail_lib.py b/site_utils/gmail_lib.py
index 2659292..c0d0c43 100755
--- a/site_utils/gmail_lib.py
+++ b/site_utils/gmail_lib.py
@@ -25,6 +25,7 @@
 from email.mime.text import MIMEText
 
 import common
+from autotest_lib.client.bin import utils
 from autotest_lib.client.common_lib import global_config
 from autotest_lib.server import site_utils
 
@@ -44,7 +45,7 @@
 try:
     from chromite.lib import metrics
 except ImportError:
-    metrics = site_utils.metrics_mock
+    metrics = utils.metrics_mock
 
 
 DEFAULT_CREDS_FILE = global_config.global_config.get_config_value(