Added Data Saver ChromeDriver test for via header fallback logic.

tools/chrome_proxy/webdriver/fallback.py

 # Copyright 2017 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 common
 from common import TestDriver
 from common import IntegrationTest
 
 class Fallback(IntegrationTest):
 
@@ skipping 18 matching lines @@
       self.assertTrue(
         test_driver.SleepUntilHistogramHasEntry("DataReductionProxy.ProbeURL"))
 
       test_driver.LoadURL('http://check.googlezip.net/test.html')
       responses = test_driver.GetHTTPResponses()
       self.assertNotEqual(0, len(responses))
       for response in responses:
           self.assertHasChromeProxyViaHeader(response)
           self.assertEqual(u'http/1.1', response.protocol)
 
+  # Verify that when Chrome receives a non-4xx response through a Data Reduction
+  # Proxy that doesn't set a proper via header, Chrome falls back to the next
+  # available proxy.
+  def testMissingViaHeaderNon4xxFallback(self):
+    with TestDriver() as test_driver:
+      test_driver.AddChromeArg('--enable-spdy-proxy-auth')
+
+      # Set the primary Data Reduction Proxy to be the test server, which does
+      # not add any Via headers. The fallback Data Reduction Proxy is set to the
+      # canonical Data Reduction Proxy target.
+      test_driver.AddChromeArg('--data-reduction-proxy-http-proxies='
+                               'https://chromeproxy-test.appspot.com;'
+                               'http://compress.googlezip.net')
+
+      # Load a page that should fall back off of the test server proxy, and onto
+      # the canonical proxy that will set the correct Via header.
+      test_driver.LoadURL('http://chromeproxy-test.appspot.com/default')
+      responses = test_driver.GetHTTPResponses()
+      self.assertNotEqual(0, len(responses))
+      for response in responses:
+        self.assertHasChromeProxyViaHeader(response)
+        self.assertEqual(u'http/1.1', response.protocol)
+
+      # Check that the BypassTypePrimary histogram has a single entry in the
+      # MissingViaHeaderOther category (which is enum value 5), to make sure
+      # that the bypass was caused by the missing via header logic and not
+      # something else.
+      histogram = test_driver.GetHistogram(
+          "DataReductionProxy.BypassTypePrimary")
+      self.assertEqual(1, histogram['count'])
+      self.assertIn({'count': 1, 'high': 6, 'low': 5}, histogram['buckets'])
+
+
 if __name__ == '__main__':
   IntegrationTest.RunAllTests()