Add a layout test for module script preloading

As a follow-up to https://chromium-review.googlesource.com/658016,
this adds a layout test that makes sure module scripts are preloaded.

Bug: 740886
Change-Id: Iffa21862444d940750d03bf7cb744812cd87f4a1
Reviewed-on: https://chromium-review.googlesource.com/664498
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501557}
2 files changed