Add 404 check to BTF Litepage test

tools/chrome_proxy/webdriver/lite_page.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
 
 import time
 
@@ skipping 52 matching lines @@
         if '/csi?' in response.url:
           continue
         if response.url.startswith('data:'):
           continue
         self.assertIn('exp=ignore_preview_blacklist',
           response.request_headers['chrome-proxy'])
         if (self.checkLitePageResponse(response)):
           lite_page_responses = lite_page_responses + 1
       self.assertEqual(1, lite_page_responses)
 
-      # Scroll to the bottom of the window and make sure there were more
-      # requests that were proxied.
-      scroll_js = 'window.scrollTo(0,Math.max(document.body.scrollHeight));'
-      test_driver.ExecuteJavascriptStatement(scroll_js)
-      # Give some time for loading after each scroll.
+      # Scroll to the bottom of the window and ensure scrollHeight increases.
+      original_scroll_height = test_driver.ExecuteJavascriptStatement(
+        'document.body.scrollHeight')
+      test_driver.ExecuteJavascriptStatement(
+        'window.scrollTo(0,Math.max(document.body.scrollHeight));')
+      # Give some time for loading after scrolling.
       time.sleep(2)
-      test_driver.ExecuteJavascriptStatement(scroll_js)
-      time.sleep(2)
+      new_scroll_height = test_driver.ExecuteJavascriptStatement(
+        'document.body.scrollHeight')
+      self.assertGreater(new_scroll_height, original_scroll_height)
+
+      # Make sure there were more requests that were proxied.
       responses = test_driver.GetHTTPResponses(override_has_logs=True)
       self.assertNotEqual(0, len(responses))
       for response in responses:
         self.assertHasChromeProxyViaHeader(response)
+        self.assertIn(response.status, [200, 204])
 
   # Checks that Lo-Fi images are used when the user is in the
   # DataCompressionProxyLitePageFallback field trial and a Lite Page is not
   # served.
   def testLitePageFallback(self):
     with TestDriver() as test_driver:
       test_driver.AddChromeArg('--enable-spdy-proxy-auth')
       test_driver.AddChromeArg('--force-fieldtrials='
                                'DataCompressionProxyLoFi/Enabled_Preview/'
                                'DataCompressionProxyLitePageFallback/Enabled')
@@ skipping 59 matching lines @@
           continue
 
         self.checkLoFiResponse(response, False)
 
       # Verify that a Lite Page was requested and that the page fell back to
       # Lo-Fi images.
       self.assertEqual(1, lite_page_requests)
 
 if __name__ == '__main__':
   IntegrationTest.RunAllTests()