Fix missed legacy_page_test module update.

This was missed by https://codereview.chromium.org/2539973004

BUG=678600

Review-Url: https://codereview.chromium.org/2615613005
Cr-Commit-Position: refs/heads/master@{#441786}

tools/chrome_proxy/common/network_metrics.py

@@ -23,7 +23,7 @@ from telemetry.page import legacy_page_test
 from telemetry.value import scalar
 
 
-class NetworkMetricException(page_test.MeasurementFailure):
+class NetworkMetricException(legacy_page_test.MeasurementFailure):
   pass