Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

This makes all of our smart pointers use nullptr_t to avoid issues when
using the oilpan transition types where we get a mix of nullptr and 0
that does not work unless we make OwnPtr, RefPtr, PassRefPtr consistently
use one or the other. Since we have decided to use nullptr for OwnPtr
(which used to use 0) this change goes all out and uses nullptr
consistently for all of them.

R=eseidel@chromium.org, haraken@chromium.org, wibling@chromium.org, abarth@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/170603003

git-svn-id: svn://svn.chromium.org/blink/trunk@167578 bbb929c8-8fbe-4397-9dbb-9b2b20218538