The WebAssembly code now uses these new APIs to allocate memory with guard
regions. Guarded array buffers are no longer always external, which eliminates
a lot of special cases around WebAssembly memory.

Bug: chromium:720302
Change-Id: I355b74ac30a05a18c8b363bd256d57458742849f
Reviewed-on: https://chromium-review.googlesource.com/505715Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45436}