service worker: Add tests for cross-origin WebVTT responses with redirects.

This is a follow-up to: https://chromium-review.googlesource.com/c/chromium/src/+/907013

Tests only; the code seems to work.

R=horo, kinuko

Bug: 808825
Change-Id: I6b86a4dac159b754cfe5e6b92ac390c07de89ab3
Reviewed-on: https://chromium-review.googlesource.com/918261
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536679}