service worker: Disallow opaque responses for WebVTT.

WebVTT loading code was determining if a response was cross-origin by looking
at request URL. But a service worker may have intercepted the request and
provided a response from cross-origin.

Bug: 808825
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Ia121c0a8eae782de93d9777603426500a9709f8c
Reviewed-on: https://chromium-review.googlesource.com/907013
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535625}
8 files changed