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This is a reland of c31de326. The CHECK
that was triggering (crbug.com/868341) has been removed but the change
is otherwise the same. The CHECK will be readded once all of the bugs
that it reveals are fixed (crbug.com/869154)

Original change's description:
> Fix invalid handle errors found with App Verifier
>
> ChildProcessData was holding a raw ProcessHandle which meant that there
> was no guarantee that the handle would be valid when it was used. This
> change switches to a ScopedHandle on Windows, with the associated
> duplication of the handle, so that it is guaranteed to be valid. This
> fixes a frequently encountered use of an invalid handle that was
> detected by Application Verifier. This change also adds a CHECK to
> base::GetProcId to check for use of invalid handles even when not using
> Application Verifier.
>
> ChildProcessData::handle is now a private member so all references to
> it were updated to use accessors.
>
> When ChildProcessData::SetHandle is called the handle is duplicated.
> When a ChildProcessData object needs to be copied an explicit
> Duplicate function is called.
>
> An IsHandleValid helper function was added.
>
> An invalid test was removed because it was triggering the CHECK in
> base::GetProcId.
>
> Bug: 417532,821453,865805
> Change-Id: I7e38140335c0140536919341f011f144f150c88f
> Reviewed-on: https://chromium-review.googlesource.com/1145767
> Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
> Reviewed-by: Jochen Eisinger <jochen@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#578419}

Bug: 417532, 821453, 865805, 868341, 869154
Change-Id: I022982f447cce4649618f4413e960f5639ad750a
Reviewed-on: https://chromium-review.googlesource.com/1155498Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579454}