Although Wasm memory indices are all unsigned, they sometimes get assembled
as 32-bit signed immediates. Values in the top half of the Wasm memory space
will then get sign extended, causing Wasm to access in front of its memory
buffer.

Usually this region is not mapped anyway, so faults still happen as they are
supposed to. This change protects this region with guard pages so we are
guaranteed to always fault when this happens.

Bug: v8:5277
Change-Id: Id791fbe2a5ac1b1d75460e65c72b5b9db2a47ee7
Reviewed-on: https://chromium-review.googlesource.com/484747
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44905}