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Use the first 16bytes region of the allocated memory for tag. So both data region, and PartitionCookie region will be moved by +16bytes.
This behavior is disabled behind ENABLE_CHECKED_PTR.

Change-Id: I09a0241f61ca1b186b239097601ea498f6d9f00c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2263417
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782887}