Add a simple fetch API test.

LayoutTests/http/tests/fetch/script-tests/simple.js

+var global = this;
+if (global.importScripts) {
+  importScripts('/resources/testharness.js');
+}
+
+promise_test(function(test) {
+    return Promise.resolve().then(function() {
+        return fetch(new Request('/fetch/resources/hello.txt'));
+      }).then(function(res) {
+        return res.text();
+      }).then(function(text) {
+        assert_equals(text, 'hello, world\n', 'response.body');
+      });
+  });
+
+done();