Expose fetchLater only in a SecureContext

[TimvdLippe]

Tested per https://github.com/web-platform-tests/wpt/blob/c009ca2fa7d311b1a257222aa0f9fd7706ffa4d6/fetch/fetch-later/non-secure.window.js

[TimvdLippe]

web-platform-tests/wpt#55123 updates the IDL test to run in a secure context to check for the existence of fetchLater. Currently Chromium browsers fail these tests: https://wpt.fyi/results/fetch/api/idlharness.any.html?label=experimental&label=master&aligned

[annevk]

cc @noamr

[noamr]

LGTM
CC @mingyc

[annevk]

This is a normative change and therefore would normally require the PR template. I'm going to waive that requirement given that Chromium is supportive and is the only implementation so far (out of Chromium, Gecko, and WebKit) and we already have test coverage. It should also be non-controversial as this API is partially permission-gated and permissions shouldn't work without HTTPS.

[annevk]

Thanks @TimvdLippe!

[noamr]

This is a normative change and therefore would normally require the PR template. I'm going to waive that requirement given that Chromium is supportive and is the only implementation so far (out of Chromium, Gecko, and WebKit) and we already have test coverage. It should also be non-controversial as this API is partially permission-gated and permissions shouldn't work without HTTPS.

Also it was gated by a security error anyway when used outside of a potentially trustworthy context. So this mainly reduces the error surface rather than changes functionality