Add layout tests for Service Worker fallbacked subresource requests.

LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-worker.js

Index: LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-worker.js
diff --git a/LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-worker.js b/LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-worker.js
index 0e12911d42219dae92650b41e34d8251fc48ddc0..aeb63eab8c70db6280b40f58e31bb5a3f7e7f263 100644
--- a/LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-worker.js
+++ b/LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-worker.js
@@ -1,7 +1,10 @@
+// TODO(horo): Service worker can be killed at some point during the test. So we
+// should use storage API instead of this global variable.
 var requests = [];
 
 self.addEventListener('message', function(event) {
     event.data.port.postMessage({requests: requests});
+    requests = [];
   });
 
 self.addEventListener('fetch', function(event) {