Perhaps we should throw an exception and/or change our external API to
use a MaybeLocal, but that would be a bigger change. For now, we just
return undefined when something goes wrong with the DebugContext,
which is good enough to avoid crashing.

BUG=chromium:474538
LOG=y

Review URL: https://codereview.chromium.org/1065213002

Cr-Commit-Position: refs/heads/master@{#27650}