The old method always returned a Handle<Object>, requiring an explicit
cast in the caller. This CL makes it return Handle<T> if called with a
T* as parameter.

Also, remove now redundant casts from callers.

R=bmeurer@chromium.org

Change-Id: I13cfb2f2e812e8582a9a1d9d6c8a5a24f40d0e79
Reviewed-on: https://chromium-review.googlesource.com/458376Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44012}