This is a reland of 4dd293d9
Original change's description:
> [Memory] Move VirtualMemory out of base:: platform.
> 
> - Moves base::VirtualMemory to v8::internal::VirtualMemory.
> - Makes VirtualMemory platform-independent by moving internals to new
>   OS:: static methods, for each platform.
> 
> This will make it easier to delegate memory management in VirtualMemory
> to V8::Platform, so that embedders like Blink can override it. We can't
> depend on V8::Platform in base/platform.
> 
> Bug: chromium:756050
> Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
> Change-Id: Iadfe230b6850bd917727a373f277afded9883adf
> Reviewed-on: https://chromium-review.googlesource.com/653214
> Commit-Queue: Bill Budge <bbudge@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48048}

Bug: chromium:756050
Change-Id: Ib492c7c69f1833be127a571808301e96b84b8aa2
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/671125
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48084}