DCHECKs (defined in src/logging.h) should not be used in third_party
code. This CL removes the only such use.

R=marja@chromium.org

Bug: v8:7754, v8:7965
Change-Id: I44196bd2d85e459e27c6a897f039f741edf1e2e1
Reviewed-on: https://chromium-review.googlesource.com/1158684Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54967}