This reverts commit 7cf29d8d.

Original change's description:
> [wasm] always allocate memory when guard regions are needed
>
> When using trap handlers, memory references do not get any checks inserted. This
> means there is no check for a null memory as happens when the memory size is
> 0. Normally this would be correctly caught as an out of bounds access, since the
> low memory addresses are not normally mapped. However, if they were mapped for
> some reason, we would not catch the out of bounds access.
>
> The fix is to ensure WebAssembly instances always have a guard region even if
> the memory is size 0.
>
> Bug: chromium:769637

Change-Id: I09fdaea92b7ccb3a6cc9e28392171ec098538a00
Reviewed-on: https://chromium-review.googlesource.com/695812
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48293}