Reland "base/allocator: Respect malloc() alignment guarantees in PartitionAlloc."

This reverts commit 25c28325359ee3ce2a640bb4367d38b7c677e6b9.

Reason for revert: Updated the failing test.

> Original change's description:
> > base/allocator: Respect malloc() alignment guarantees in PartitionAlloc.
> >
> > Generally speaking, malloc() must return data suitably aligned for
> > std::max_align_t. In practice this means 16 bytes on 64 bit
> > architectures, whereas PartitionAlloc only provides 8 there. This is
> > problematic to use it as a malloc() replacement.
> >
> > This CL makes PartitionAlloc respect the alignment guarantees on 64 bit
> > architectures.
> >
> > Bug: 787153,998048
> > Change-Id: I860f9d0439d59f8d494306452b8cdbff515c8300
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219498
> > Commit-Queue: Benoit L <lizeb@chromium.org>
> > Reviewed-by: Kentaro Hara <haraken@chromium.org>
> > Reviewed-by: Takashi Sakamoto <tasak@google.com>
> > Cr-Commit-Position: refs/heads/master@{#772662}
>
> TBR=tasak@google.com,haraken@chromium.org,lizeb@chromium.org
>
> Change-Id: Ic3266b40748d22cb25d91ab041db79da42851f4f
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 787153, 998048
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2220085
> Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#772669}

# Not skipping CQ checks because this is a reland.

Bug: 787153, 998048
Change-Id: I15257083b8e651c2ec0516563e5b8728bc5f158d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218280
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772748}