This change adds an addition 2GiB of guard region in front of guarded
WebAssembly memories. This provides defense in depth against code
generation errors, such as accidental sign extension of a 32-bit value.
As we consider supporting 4GiB WebAssembly memories, this is an
important line of defense.

Change-Id: Ica643c01ef28b5d2a01fbbd46273442545d8448e
Reviewed-on: https://chromium-review.googlesource.com/1108503
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53949}