- 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}