The ptr_to_* methods do (often unnecessary) type checks, and can
return nullptr. This is problematic since the handlified getter
uses them, and assumes the result to be non-null. So change
them to only to a DCHECK and never return nullptr, and introduce
maybe_ptr_to_* with the old semantics.

R=titzer@chromium.org, ahaas@chromium.org

Review-Url: https://codereview.chromium.org/2509053003
Cr-Commit-Position: refs/heads/master@{#41079}