SharedWorker: Fix the wrong WPT undefined-arguments.html

The worker's name must be |''| in this test.

According to the spec, the default value of WorkerOptions (which is the
second argument of SharedWorker) is set as follows:
dictionary WorkerOptions {
  WorkerType type = "classic";
  RequestCredentials credentials = "same-origin";
  DOMString name = "";
};

spec:https://html.spec.whatwg.org/multipage/workers.html#workeroptions
Here, name property is set to an empty string.
Therefore, the worker's name must be an empty string when the second
argument is |undefined|.

Bug: 907749
Change-Id: I5bc9c5afcf76beae1e3924661b2ea65fad5efe1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015911
Commit-Queue: Eriko Kurimoto <elkurin@google.com>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734364}

third_party/blink/web_tests/external/wpt/workers/constructors/SharedWorker/undefined-arguments-expected.txt

-This is a testharness.js-based test.
-FAIL undefined as arguments assert_equals: second arg expected "undefined" but got ""
-Harness: the test ran to completion.
-

third_party/blink/web_tests/external/wpt/workers/constructors/SharedWorker/undefined-arguments.html

@@ -14,7 +14,7 @@ onconnect = function(e) {
   var worker = new SharedWorker(undefined, undefined);
   worker.port.addEventListener('message', this.step_func(function(e) {
     assert_equals(e.data[0], 'undefined', 'first arg');
-    assert_equals(e.data[1], 'undefined', 'second arg');
+    assert_equals(e.data[1], '', 'second arg');
     this.done();
   }), false);
   worker.port.start();
@@ -22,4 +22,4 @@ onconnect = function(e) {
 </script>
 <!--
 */
-//-->
\ No newline at end of file
+//-->