On 32bit windows, DiscardSystemPages fails sometimes (see bug). We don't
have a reproducer, so this is a purely speculative fix.
{SetPermissions} has some platform-specific logic to also discard the
pages. Specifially, on windows it uses {VirtualFree} instead of
{DiscardVirtualMemory}.
{SetPermissions} is also semantically stronger, since it forbids any
further access to the pages.

R=mstarzinger@chromium.org

Bug: v8:8217, chromium:960707
Change-Id: I8b3325264c86aff8d6e7c6b01c22ae410e87faf4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1601134Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61321}