- Memory.Grow with guard pages enabled should adjust amount of allocated
   memory, and not allocate a new buffer. This was disabled because previously
   the backing store was freed in the MemoryFinalizer, and we needed to be sure
   that the backing store is not released till the last buffer using it is
   released. This is now safe as we no longer use the MemoryFinalizer
 - SetProtection should use Guard/Unprotect that use mprotect underneath,
   instead of CommitRegion/UncommitRegion that use mmap
 - Move buffer allocation to the end to avoid inconsistent memory due to GC

BUG=v8:5886

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I0d7edb884bd1e3167eb5fbced6953c6401688d40
Reviewed-on: https://chromium-review.googlesource.com/629517Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47960}