Casting to HeapObject before checking whether the object actually is
a HeapObject is undefined behavior.

Bug: chromium:738743
Change-Id: I7be8dfbc18203c6be008af73549a915f9b6bd3de
Reviewed-on: https://chromium-review.googlesource.com/680768Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48128}