We sometimes allow allocation to fail and return a null Handle in that
case (e.g. for grow_memory). This refactors this code to return a
MaybeHandle instead, to document that allocation might fail and to force
the caller to handle this.

R=mstarzinger@chromium.org

Change-Id: Ia3ba65f840cfb1cf93e8dbd508a17375c19bae58
Reviewed-on: https://chromium-review.googlesource.com/995438
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52358}