Add an about:flag to support alternative data saver features

tools/chrome_proxy/webdriver/smoke.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
 import time
 from common import TestDriver
 from common import IntegrationTest
 from decorators import NotAndroid
 
 
 class Smoke(IntegrationTest):
 
+  # Checks that the "alternative" value is sent in the "exp" directive.
+  def testAlternative(self):
+    # If it was attempted to run with another experiment, skip this test.
+    with TestDriver() as test_driver:
+      test_driver.AddChromeArg('--enable-spdy-proxy-auth')
+      test_driver.AddChromeArg('--data-reduction-proxy-lo-fi=always-on')
+      test_driver.AddChromeArg('--enable-data-reduction-proxy-lite-page')
+      test_driver.AddChromeArg(
+        '--enable-features=DataReductionProxyAlternativeBackEnd')
+
+      test_driver.LoadURL('http://check.googlezip.net/test.html')
+
+      # Alternative does not represent a strict implementation, but must have
+      # "exp=alternative".
+      for response in test_driver.GetHTTPResponses():
+        self.assertHasChromeProxyViaHeader(response)
+        self.assertEqual(200, response.status)
+        if not common.ParseFlags().browser_args or (
+          '--data-reduction-proxy-experiment' not in
+          common.ParseFlags().browser_args):
+            self.assertIn('exp=alternative',
+              response.request_headers['chrome-proxy'])
+
   # Ensure Chrome does not use DataSaver in Incognito mode.
   # Clank does not honor the --incognito flag.
   @NotAndroid
   def testCheckPageWithIncognito(self):
     with TestDriver() as t:
       t.AddChromeArg('--enable-spdy-proxy-auth')
       t.AddChromeArg('--incognito')
       t.LoadURL('http://check.googlezip.net/test.html')
       for response in t.GetHTTPResponses():
         self.assertNotHasChromeProxyViaHeader(response)
@@ skipping 85 matching lines @@
       t.LoadURL('http://check.googlezip.net/static')
       # http://check.googlezip.net/static is a test page that has
       # image/css/javascript resources.
       responses = t.GetHTTPResponses()
       self.assertNotEqual(0, len(responses))
       for response in responses:
         self.assertHasChromeProxyViaHeader(response)
 
 if __name__ == '__main__':
   IntegrationTest.RunAllTests()