Update reference to page_test module to use legacy_page_test module

tools/chrome_proxy/live_tests/chrome_proxy_metrics.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import logging
 import time
 
 from common import chrome_proxy_metrics
 from common import network_metrics
 from common.chrome_proxy_metrics import ChromeProxyMetricException
-from telemetry.page import page_test
+from telemetry.page import legacy_page_test
 from telemetry.value import scalar
 
 
 class ChromeProxyMetric(network_metrics.NetworkMetric):
   """A Chrome proxy timeline metric."""
 
   def __init__(self):
     super(ChromeProxyMetric, self).__init__()
     self.compute_data_saving = True
 
@@ skipping 105 matching lines @@
 
     request_duration = (float(response_start) - request_start)
     results.AddValue(scalar.ScalarValue(
         results.current_page, 'request_duration', 'ms', request_duration,
         important=False))
 
     response_duration = (float(response_end) - response_start)
     results.AddValue(scalar.ScalarValue(
         results.current_page, 'response_duration', 'ms', response_duration,
         important=False))