Remove an unnecessary reinterpret_cast from a persistent to an handle

It's really only required for debug checks, and in debug builds, we can
take the hit of creating a safe handle

BUG=none
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/670483003

git-svn-id: svn://svn.chromium.org/blink/trunk@184000 bbb929c8-8fbe-4397-9dbb-9b2b20218538