- Adds overload to v8::Platform that will make it easier for embedders to
  maintain a reserve of address space for large, contiguous allocations.
- Rewrites retry logic using loops.
- Moves retry logic from some VirtualMemory allocation functions to AllocPages.

Bug: chromium:756050
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I52e66f9f8b15b6ce2a2f36e74783f178b8cd5cf7
Reviewed-on: https://chromium-review.googlesource.com/840724
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50303}