This is a convenience API that an embedder can use to do final checks on
the return value. Note that this creates a new handle and thus defeats
the performance optimization done for ReturnValue - an embedder should
only use this in non-performance critical code paths.

BUG=
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/1875263003

Cr-Commit-Position: refs/heads/master@{#35409}