Add null checks in navigator.serviceWorker access to fix possible crash

LayoutTests/fast/serviceworker/access-container-with-invalid-context.html

+<script src="../../resources/js-test.js"></script>
+<script>
+description("This test checks if accessing navigator.serviceWorker doesn't fail with invalid frame");
+
+if (window.testRunner) {
+  testRunner.dumpAsText();

[dominicc (has gone to gerrit), 2014/03/31 04:52:09]

I could be wrong, but isn't dumpAsText the default?

[kinuko, 2014/03/31 12:33:05]

I removed this as I've converted it to use w3c test harness.

+  testRunner.setCanOpenWindows();

[dominicc (has gone to gerrit), 2014/03/31 04:52:09]

Do you need separate windows? Why not just use an iframe?

[kinuko, 2014/03/31 12:33:05]

Hmm, on my local env I keep failing to repro the crash with iframe. (Is there a
known test pattern I could try?)

+  testRunner.waitUntilDone();
+}
+
+var callbackCount = 0;
+function callback() {
+  ++callbackCount;
+  if (callbackCount == 100) {
+    debug("DONE");
+    testRunner.notifyDone();
+    return;
+  }
+  try {
+    // window.navigator is replaced with a new window's one, which could
+    // be now invalid.
+    var sw = navigator.serviceWorker;
+  } catch (error) {}
+  var w = window.open("x", "y");
+  window.navigator = w.navigator;
+  setTimeout(callback, 1);
+}
+
+setTimeout(callback, 1);

[dominicc (has gone to gerrit), 2014/03/31 04:52:09]

Why do it 100 times? Do you just need a strategic gc() in here or something?

[kinuko, 2014/03/31 12:33:05]

Either with gc() or not I can't seem to reliably repro this (::detachClient one
is reproducible but the other one is not).

I removed the ad-hoc repetition from this test code anyway.

+</script>