[css-paint-api] Restrict paint() function to only parse in secure contexts.

This is a direct followup to:
https://chromium-review.googlesource.com/c/chromium/src/+/759112

As per the blink-dev decision:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/Jex3idOld48

Bug: 578252
Change-Id: I865d25b15581adb688e94788d05f78e0974858c8
Reviewed-on: https://chromium-review.googlesource.com/780961
Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518491}
11 files changed