VirtualMemory objects can be moved since https://crrev.com/c/1213062,
so there is no need any more to return them via pointer argument. This
also makes the {AllocVirtualMemory} and {AlignedAllocVirtualMemory}
functions superfluous.

R=ishell@chromium.org, titzer@chromium.org

Bug: v8:8015
Change-Id: Id72921e1c66a6c10be6647194603b8283e010e24
Reviewed-on: https://chromium-review.googlesource.com/1226972Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55947}