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This CL 'zaps' (`memset`s) the allocated region on free. This may
help detect some instances of UAF, causing them to crash (with
nullptr deref).

Rather than doing so on every free, we zap probabilistically to
reduce the performance impact.

If the perf bots show an unacceptable regression, we'll revert this.
But, it might be acceptable.

Bug: 1005070, 1013329, 1013326, 1013324
Change-Id: Ia36ee17625290646216985dd6fdffabfa84a9ba2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1873272
Commit-Queue: Chris Palmer <palmer@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816834}