sandwich: Fix some trivial failures on SWR benchmark runs

tools/android/loading/sandwich_swr.py

 # Copyright 2016 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.
 
 """ This module implements the Stale-While-Revalidate performance improvement
 experiment on third parties' resources.
 
 The top level operations of the experiment are:
   1. Record WPR archive;
   2. Create a patched WPR archive so that all resource are getting cached;
@@ skipping 78 matching lines @@
     chrome_cache.UnzipDirectoryContent(original_cache_archive_path, cache_path)
     cache_backend = chrome_cache.CacheBackend(cache_path, 'simple')
     cache_keys = set(cache_backend.ListKeys())
     for request in trace.request_track.GetEvents():
       if request.url not in cache_keys:
         continue
       if request.url in urls_should_not_be_cached:
         cache_backend.DeleteKey(request.url)
         delete_count += 1
         continue
+      if not request.HasReceivedResponse():
+        continue
       if request.url in urls_to_enable_swr:
         request.SetHTTPResponseHeader(
             'cache-control', 'max-age=0,stale-while-revalidate=315360000')
         request.SetHTTPResponseHeader(
             'last-modified', 'Thu, 23 Jun 2016 11:30:00 GMT')
         swr_patch_count += 1
       elif request.url in urls_already_with_swr:
         # Force to use SWR on resources that originally attempted to use it.
         request.SetHTTPResponseHeader(
             'cache-control', 'max-age=0,stale-while-revalidate=315360000')
@@ skipping 203 matching lines @@
 
       run_metrics_list.sort(key=lambda e: e['repeat_id'])
       with open(ExtractMetrics.path, 'w') as csv_file:
         writer = csv.DictWriter(csv_file, fieldnames=(additional_column_names +
                                     sandwich_metrics.COMMON_CSV_COLUMN_NAMES))
         writer.writeheader()
         for run_metrics in run_metrics_list:
           writer.writerow(run_metrics)
 
     self._common_builder.default_final_tasks.append(ExtractMetrics)