[Native File System] Limit entire API surface to secure contexts.

Also verify in browser process that all access is from potentially
trustworthy origins.

Bug: 971408
Change-Id: Iab770c831be67603a728e8aa55229b4048a410f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1699442
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677876}
14 files changed