Add web platform tests about what credentials mode is used for ES6 module fetching for various configurations

Bug: 717525
Change-Id: I4f399ae51e8211274485e3d29dbf8f48ddf18dc4
Reviewed-on: https://chromium-review.googlesource.com/551519
Commit-Queue: Takeshi Yoshino <tyoshino@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483307}
3 files changed