Add a test to verify that sync XHR doesn't timeout

Blink had a bug that sync XHR would timeout after 10 seconds. Add a
regression test to make sure the issue does not reoccur.

BUG=844268

Change-Id: I161e133582b719102a02349828cd4d9a08773785
Reviewed-on: https://chromium-review.googlesource.com/1070089
Commit-Queue: Adam Rice <ricea@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561400}

third_party/WebKit/LayoutTests/external/wpt/xhr/sync-no-timeout.any.js

+// META: global=sharedworker
+// META: timeout=long
+
+// This is a regression test for https://crbug.com/844268, when a timeout of 10
+// seconds was applied to XHR in Chrome. There should be no timeout unless the
+// "timeout" parameter is set on the object.
+test(t => {
+  let xhr = new XMLHttpRequest();
+
+  // For practical reasons, we can't wait forever. 12 seconds is long enough to
+  // reliably reproduce the bug in Chrome.
+  xhr.open('GET', 'resources/trickle.py?ms=1000&count=12', false);
+
+  // The test will fail if this throws.
+  xhr.send();
+}, 'Sync XHR should not have a timeout');