We were just debugging a trap triggered in the c entry stub, because
there was a pending exception even though the return value of the
runtime function was not {heap()->exception()}.
The DCHECKs added here help to catch this common error earlier.

R=mstarzinger@chromium.org
CC=ahaas@chromium.org

Also-by: ahaas@chromium.org
Change-Id: I3718dcb64a7924137c9f4bddff62776772d10386
Reviewed-on: https://chromium-review.googlesource.com/594447Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47031}