sandwich: Fix some trivial failures on SWR benchmark runs

tools/android/loading/sandwich_swr.py

Index: tools/android/loading/sandwich_swr.py
diff --git a/tools/android/loading/sandwich_swr.py b/tools/android/loading/sandwich_swr.py
index a902f4da851bcc1a0535526897dabad8f71a116c..e6b3736b50bd93b844295b8b9ed9180ffad89ef5 100644
--- a/tools/android/loading/sandwich_swr.py
+++ b/tools/android/loading/sandwich_swr.py
@@ -96,6 +96,8 @@ def _BuildBenchmarkCache(
         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')