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When --allow-sandbox-debugging is used, we set-up a new signal
handler in the Zygote (inherited by all renderers) that performs a
chroot().

This allows testing of sandbox violation crash report with official
binaries shipping to users.

BUG=415842
R=mdempsky@chromium.org

Review URL: https://codereview.chromium.org/585123003

Cr-Commit-Position: refs/heads/master@{#295826}