Add layout tests for Service Worker fallbacked subresource requests.

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

 var requests = [];

[falken, 2015/08/11 09:15:33]

Technically this isn't portable since the sw can be killed at some point during
the test. I wonder how difficult it would be to just use the cachestorage API to
store these requests. At least we need a TODO.

[horo, 2015/08/12 11:28:10]

Added TODO

 
 self.addEventListener('message', function(event) {
     event.data.port.postMessage({requests: requests});
+    requests = [];
   });
 
 self.addEventListener('fetch', function(event) {
     requests.push({
         url: event.request.url,
         mode: event.request.mode
       });
   });