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This reverts commit 0319eb27.

Reason for revert: Rationale for this change was incorrect; browser-side DiscardableSharedMemory objects _always_ have their handles closed.

Original change's description:
> [base] Don't try to decommit pages if they're about to be unmapped.
> 
> DiscardableSharedMemory::Purge() attempts to proactively discard pages
> to prevent slow-to-respond clients from keeping the pages live longer.
> 
> If the instance is in-process then these kernel calls are unnecessary
> since the memory is about to be unmapped anyway.
> 
> Bug: 1023693
> Change-Id: I2bc19eb64f3fdc009c5bf8c4ed6369b2b5081045
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911487
> Auto-Submit: Wez <wez@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Commit-Queue: Wez <wez@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#714684}

TBR=dcheng@chromium.org,wez@chromium.org

Change-Id: I2b451e314a1ee73757cad5c492d6fd550eb83a09
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1023693
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913599Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714728}