This is a reland of b2f8280e, fixing
asan and msan invocations to use {Segment::capacity()} instead of
{Segment::size()}.

Original change's description:
> [zone] Teach ASan about the zone segment pool
>
> This adds proper poisoning/unpoisoning to segments put into the segment
> pool of an accounting allocator, and also marks a segment uninitialized
> when returning it from the pool. This gives ASan a better chance at
> catching use-after-free and others.
>
> Drive-by: Fix type check in ASAN_POISON_MEMORY_REGION
>
> R=mstarzinger@chromium.org
>
> Change-Id: Iadbdd7c0a0c80da8e7b9bb8f3399209715436073
> Reviewed-on: https://chromium-review.googlesource.com/c/1489086
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#59932}

Change-Id: I3150a7f96fefd94bd96af2fdc24c302a4a5d0ea3
Reviewed-on: https://chromium-review.googlesource.com/c/1493866Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59937}