Add 404 check to BTF Litepage test

Using a local proxy instance with the associated bug, the failed
response gave a 404 status. This check differentiates between the bug
and not.

BUG=715644

Review-Url: https://codereview.chromium.org/2844403002
Cr-Commit-Position: refs/heads/master@{#468106}

tools/chrome_proxy/webdriver/lite_page.py

@@ -70,18 +70,23 @@ class LitePage(IntegrationTest):
           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.
-      time.sleep(2)
-      test_driver.ExecuteJavascriptStatement(scroll_js)
+      # 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)
+      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