ResourcePtr refactoring

- ResourcePtrBase destructor is non-virtual, so make it protected.
- ResourcePtr<R> should accept ResourcePtr<U> only when U* is convertible to R*.
- Use static_cast instead of reinterpret_cast.
- Delete the unused assignment operator.

BUG=None
R=tyoshino

Review URL: https://codereview.chromium.org/720693002

git-svn-id: svn://svn.chromium.org/blink/trunk@185271 bbb929c8-8fbe-4397-9dbb-9b2b20218538