Delay generation of User-Agent header to URLRequestHttpJob and accept custom User-Agent from XHR/Fe…

third_party/WebKit/LayoutTests/http/tests/inspector/service-workers/resources/user-agent-override-worker.js

 function dumpUserAgent()
 {
-    fetch('http://127.0.0.1:8000/inspector/resources/echo-headers.php')
+    fetch('http://127.0.0.1:8000/resources/echo-headers.php')
         .then(function(response) {
             return response.text();
         }).then(function(body) {
             var userAgentLine = /HTTP_USER_AGENT:.*/.exec(body)[0];
             console.log(userAgentLine);
         }).catch(function(err) {
             console.log('fetch failed: ' + err.message);
         });
 }
 
 self.onmessage = function(event) {
     dumpUserAgent();
 };