Add an about:flag to support alternative data saver features

tools/chrome_proxy/webdriver/smoke.py

Index: tools/chrome_proxy/webdriver/smoke.py
diff --git a/tools/chrome_proxy/webdriver/smoke.py b/tools/chrome_proxy/webdriver/smoke.py
index ff99fdf21d8af5246cb94ce336be023f259fb6e2..fab1e40408209d36860ea1c577495ebcc10be1d9 100644
--- a/tools/chrome_proxy/webdriver/smoke.py
+++ b/tools/chrome_proxy/webdriver/smoke.py
@@ -11,6 +11,29 @@ 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