Remove unnecessary reinterpret_casts in core

I replaced reinterpret_cast with static_cast for down casting, which
is at least a little safer. A couple of void* -> void* were removed entirely.

R=crogers@google.com, esprehn@chromium.org

Review URL: https://codereview.chromium.org/19275005

git-svn-id: svn://svn.chromium.org/blink/trunk@154312 bbb929c8-8fbe-4397-9dbb-9b2b20218538