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Due to a bug, the sandox_flags computed from the browser process were
taking into account the Content-Security-Policy:sandbox of the original
response, even for error pages.

This fixes the bug and add regressions tests.

Bug: 1158370,1158306
Change-Id: Idc1c575b0a4e578542c66b3f7153c6dac5d1e43d
Fixed: 1158306
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587047
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837035}