By default zx_vmo_create() creates resizable VMOs, which may be
hazardous in some cases. It's safer to use ZX_VMO_NON_RESIZABLE unless
VMO needs to be resizable. It doesn't make much difference in
OS::Allocate() because it drops the VMO handle immediately after
mapping it, still it's better to use ZX_VMO_NON_RESIZABLE for
consistency.

Change-Id: I688ee44b08042a9df7e3fae0b1b3298271b53b1c
Reviewed-on: https://chromium-review.googlesource.com/c/1277605Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56788}