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This change simply allows a valid Handle to be duplicated after
ChildProcessSecurityPolicyImpl::Remove() has been called for the process
associated with the Handle. Handles are intended to keep security state
alive beyond the process lifetime and some Handles are duplicated so
they can be passed down into lower level code. This change fixes a bug
where Duplicate() would generate invalid Handle objects if it was
called after CPSPI::Remove() was called. These calls can happen because
of Mojo message queuing and no ordering constraints between different
Mojo interfaces.

Bug: 977169, 1086306
Change-Id: Icacaf06b049ba88ae7152f5c7307312d73134824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2226156Reviewed-by: Aaron Colwell <acolwell@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Aaron Colwell <acolwell@chromium.org>
Auto-Submit: Aaron Colwell <acolwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775272}