AppCache: Rename whitelist -> allowlist in web_tests.

Bug: 1097230
Change-Id: Iaf1cd137500d3a5d9c96bc2ffaeb4a8ee1070c17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2256606


Auto-Submit: Victor Costan <pwnall@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780831}