Missed legacy_page_test module update

Continuation of cl/2615613005, updating module page_test to legacy_page_test.

BUG=678600
TBR=bustamante

Review-Url: https://codereview.chromium.org/2614123003
Cr-Commit-Position: refs/heads/master@{#441941}

tools/chrome_proxy/common/chrome_proxy_metrics.py

@@ -13,7 +13,7 @@ from telemetry.value import scalar
 CHROME_PROXY_VIA_HEADER = 'Chrome-Compression-Proxy'
 
 
-class ChromeProxyMetricException(page_test.MeasurementFailure):
+class ChromeProxyMetricException(legacy_page_test.MeasurementFailure):
   pass
 
 
@@ -115,4 +115,3 @@ class ChromeProxyResponse(network_metrics.HTTPResponse):
 
   def HasChromeProxyPassThroughRequest(self):
     return self.HasRequestHeader('Chrome-Proxy-Accept-Transform', "identity")
-