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This experimental build option to mark binaries as compatible with Shadow
Stack of Control-flow Enforcement Technology (CET).
THe option is not any close to enable in official builds, it's here
for convenice of evaluation.

As it passes base_unittests and components_unittests.

Bug: 1136224
Change-Id: I6fcd61c2e76066b444cc37c88228273838300243
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454988Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819145}