Expose Fetch API related classes to Window and WorkerGlobalScope.

LayoutTests/http/tests/fetch/resources/request.js

+var global = this;
+if (global.importScripts) {
+    // Worker case
+    importScripts('/resources/testharness.js');
+}
+
+test(function(test) {
+    var request = new Request('');
+    assert_equals(request.constructor, Request);
+}, 'Create request');
+
+test(function(test) {
+    var response = new Response;
+    assert_equals(response.constructor, Response);
+}, 'Create response');
+
+test(function(test) {
+    var headers = new Headers;
+    assert_equals(headers.constructor, Headers);
+}, 'Create headers');
+
+if (global.importScripts) {
+    // Worker case
+    done();
+}