Added Data Saver ChromeDriver test for via header fallback logic.

tools/chrome_proxy/webdriver/fallback.py

Index: tools/chrome_proxy/webdriver/fallback.py
diff --git a/tools/chrome_proxy/webdriver/fallback.py b/tools/chrome_proxy/webdriver/fallback.py
index 4de3956fc74b4c67565be897fd2cefa3037c7827..86025e2b7243aeafd493a769e94985fa9adf042b 100644
--- a/tools/chrome_proxy/webdriver/fallback.py
+++ b/tools/chrome_proxy/webdriver/fallback.py
@@ -36,5 +36,38 @@ class Fallback(IntegrationTest):
           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()