ServiceWorker: Add WPT to check if CSP sandbox is respected

Bug: 771815
Change-Id: I11dc3cf67e3e40465f612f71fc318fd7061b6581
Reviewed-on: https://chromium-review.googlesource.com/915683
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540467}