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This change introduces a Handle object so that Mojo services can
preserve the security state beyond the lifetime of the
RenderProcessHostImpl object. This allows consistent security
checks to occur even during the period when the renderer process is
shutting down and there are still pending Mojo operations in flight.
This will be used to remove all remaining uses of
ChildProcessSecurityPolicyImpl::HasSecurityState() in follow-up CLs.

- Implements new Handle object that allows security checks to provide
  consistent results after ChildProcessSecurityPolicyImpl::Remove() is
  called.
- Convert blob code to use Handle instead of the HasSecurityState()
  workaround.

This is an updated version of https://crrev.com/c/1534368 . Further
discussion of the history and reasons for this CL can be found there.

Bug: 1035399, 943887
Change-Id: I6165fad4308643a1ddc845690443e8efceac65f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1975165Reviewed-by: Aaron Colwell <acolwell@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Aaron Colwell <acolwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732296}