[Private Network Access] Use PolicyContainer for secure context bit.

This is now possible thanks to PolicyContainerNavigationBundle storing
the parent document's policies. It also happens to fix the handling of
`data:` URLs, breaking with the Blink implementation.

This change revealed a bug in the implementation of secure context for
failed navigations / error pages. They previously seemed to conserve the
secure context bit of their initiator, whereas they are now never
considered secure. See also: crbug.com/1180140#c2

Bug: chromium:1170335, chromium:1180140
Change-Id: Ib855c1be0400de41512919ae4f6ea3e10680814c
Fixed: chromium:1168024
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2672779
Commit-Queue: Titouan Rigoudy <titouan@chromium.org>
Auto-Submit: Titouan Rigoudy <titouan@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Antonio Sartori <antoniosartori@chromium.org>
Cr-Commit-Position: refs/heads/master@{#858182}