Add a simple fetch API test.

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

Index: LayoutTests/http/tests/fetch/script-tests/simple.js
diff --git a/LayoutTests/http/tests/fetch/script-tests/simple.js b/LayoutTests/http/tests/fetch/script-tests/simple.js
new file mode 100644
index 0000000000000000000000000000000000000000..76652f660d33a500922bcea597df6595e53f4e43
--- /dev/null
+++ b/LayoutTests/http/tests/fetch/script-tests/simple.js
@@ -0,0 +1,16 @@
+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();