Note that {nullptr} is a dangerous sentinel value in V8's object model
because it can be interpreted as Smi(0) and hence will turn into a
completely different type than the declared return type at runtime.

R=ahaas@chromium.org
BUG=v8:7509

Change-Id: I89cffa1160a3bf6853f91c04fb90c74ad08888a3
Reviewed-on: https://chromium-review.googlesource.com/948907Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51757}