[ServiceWorker] Set FetchEvent's request's headers' guard to immutable

LayoutTests/http/tests/serviceworker/resources/request-end-to-end-worker.js

Index: LayoutTests/http/tests/serviceworker/resources/request-end-to-end-worker.js
diff --git a/LayoutTests/http/tests/serviceworker/resources/request-end-to-end-worker.js b/LayoutTests/http/tests/serviceworker/resources/request-end-to-end-worker.js
index 199e586990fb24d2362f595a5726793bbc54c753..6e2a8bfb14df6d2cf01db44e993c61a8e35852e3 100644
--- a/LayoutTests/http/tests/serviceworker/resources/request-end-to-end-worker.js
+++ b/LayoutTests/http/tests/serviceworker/resources/request-end-to-end-worker.js
@@ -13,11 +13,18 @@ onfetch = function(e) {
     var key = header[0], value = header[1];
     headers[key] = value;
   }
+  var errorNameWhileAddingHeader = '';
+  try {
+    e.request.headers.append('Test-Header', 'TestValue');
+  } catch (e) {
+    errorNameWhileAppendingHeader = e.name;
+  }
   port.postMessage({
       url: e.request.url,
       method: e.request.method,
       referrer: e.request.referrer,
       headers: headers,
-      headerSize: e.request.headers.size
+      headerSize: e.request.headers.size,
+      errorNameWhileAppendingHeader: errorNameWhileAppendingHeader
     });
 };