Update reference to page_test module to use legacy_page_test module

tools/chrome_proxy/common/chrome_proxy_measurements.py

 # Copyright 2014 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 base64
 import logging
 
 from common import chrome_proxy_metrics as metrics
 from telemetry.core import exceptions
-from telemetry.page import page_test
+from telemetry.page import legacy_page_test
 
 
 def WaitForViaHeader(tab, url="http://check.googlezip.net/test.html"):
   """Wait until responses start coming back with the Chrome Proxy via header.
 
   Poll |url| in |tab| until the Chrome Proxy via header is present in a
   response.
 
   This function is useful when testing with the Data Saver API, since Chrome
   won't actually start sending requests to the Data Reduction Proxy until the
@@ skipping 33 matching lines @@
   tab.WaitForJavaScriptExpression('performance.timing.loadEventEnd', 60)
 
   expected_via_header = metrics.CHROME_PROXY_VIA_HEADER
   if ChromeProxyValidation.extra_via_header:
     expected_via_header = ChromeProxyValidation.extra_via_header
 
   tab.WaitForJavaScriptExpression(
       'PollDRPCheck("%s", "%s")' % (url, expected_via_header), 60)
 
 
-class ChromeProxyValidation(page_test.PageTest):
+class ChromeProxyValidation(legacy_page_test.LegacyPageTest):
   """Base class for all chrome proxy correctness measurements."""
 
   # Value of the extra via header. |None| if no extra via header is expected.
   extra_via_header = None
 
   def __init__(self, restart_after_each_page=False, metrics=None,
                clear_cache_before_each_run=True):
     super(ChromeProxyValidation, self).__init__(
         needs_browser_restart_after_each_page=restart_after_each_page,
         clear_cache_before_each_run=clear_cache_before_each_run)
@@ skipping 33 matching lines @@
           tab, results, ChromeProxyValidation.extra_via_header)
     self.AddResults(tab, results)
 
   def AddResults(self, tab, results):
     raise NotImplementedError
 
   def StopBrowserAfterPage(self, browser, page):  # pylint: disable=W0613
     if hasattr(page, 'restart_after') and page.restart_after:
       return True
     return False