Fix broken import in web_components_unittest.py.

module_test_case.py was removed from trace-viewer.
See the bug for more details.

TBR=nduca@chromium.org
BUG=470463

Review URL: https://codereview.chromium.org/1033933002

Cr-Commit-Position: refs/heads/master@{#322141}

tools/telemetry/telemetry/web_components/web_components_unittest.py

@@ -6,10 +6,10 @@ import os
 import unittest
 
 from telemetry.web_components import web_components_project
-from tvcm import module_test_case
 
 
 def DISABLED_load_tests(_, _2, _3):
+  from tvcm import module_test_case
   project = web_components_project.WebComponentsProject()
   if os.getenv('NO_TVCM'):
     suite = unittest.TestSuite()