Add Data Saver ChromeDriver test for MissingViaHeader4xx bypass behavior

tools/chrome_proxy/webdriver/bypass.py

Index: tools/chrome_proxy/webdriver/bypass.py
diff --git a/tools/chrome_proxy/webdriver/bypass.py b/tools/chrome_proxy/webdriver/bypass.py
index 3b7798ae6c16c719aacbf11045b8b13656ca37da..6000687d46641bb277526de559b93583de2c2982 100644
--- a/tools/chrome_proxy/webdriver/bypass.py
+++ b/tools/chrome_proxy/webdriver/bypass.py
@@ -102,6 +102,39 @@ class Bypass(IntegrationTest):
       for response in responses:
         self.assertHasChromeProxyViaHeader(response)
 
+  # Verify that when Chrome receives a 4xx response through a Data Reduction
+  # Proxy that doesn't set a proper via header, Chrome bypasses all proxies and
+  # retries the request over direct.
+  def testMissingViaHeader4xxBypass(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.
+      test_driver.AddChromeArg('--data-reduction-proxy-http-proxies='
+                               'https://chromeproxy-test.appspot.com;'
+                               'http://compress.googlezip.net')
+
+      # Load a page that will come back with a 4xx response code and without the
+      # proper via header. Chrome should bypass all proxies and retry the
+      # request.
+      test_driver.LoadURL(
+          'http://chromeproxy-test.appspot.com/default?respStatus=414')
+      responses = test_driver.GetHTTPResponses()
+      self.assertNotEqual(0, len(responses))
+      for response in responses:
+        self.assertNotHasChromeProxyViaHeader(response)
+        self.assertEqual(u'http/1.1', response.protocol)
+
+      # Check that the BlockTypePrimary histogram has a single entry in the
+      # MissingViaHeader4xx category (which is enum value 4), to make sure that
+      # the bypass was caused by the missing via header logic and not something
+      # else.
+      histogram = test_driver.GetHistogram(
+          "DataReductionProxy.BlockTypePrimary")
+      self.assertEqual(1, histogram['count'])
+      self.assertIn({'count': 1, 'high': 5, 'low': 4}, histogram['buckets'])
+
 
 if __name__ == '__main__':
   IntegrationTest.RunAllTests()