marja already introduced a std::is_base_of check in one of the Handle
constructors. This CL uses this check for all templatized constructors
in Handle and MaybeHandle instead of the current pointer assignment
hack.

R=marja@chromium.org, mstarzinger@chromium.org

Change-Id: I0bdd77ccff4e95015e3b82e2db782a3ec57654fe
Reviewed-on: https://chromium-review.googlesource.com/453480
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43780}