Impose strict MIME-type checks on `importScripts()`.

Intent to Remove: https://groups.google.com/a/chromium.org/d/msg/blink-dev/35t5cJQ3J_Q/FH45dl0vAwAJ

Bug: 794548, 879994
Change-Id: Ie87aedd0027921960ff429d5d8ecf168572c82de
Reviewed-on: https://chromium-review.googlesource.com/1199068
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588364}
4 files changed