Remove unused WebPrivatePtr::operator=(std::nullptr_t).

With std::nullptr_t constructors in place for the various Ptr types,
this method is no longer needed.

R=
BUG=

Review URL: https://codereview.chromium.org/177243002

git-svn-id: svn://svn.chromium.org/blink/trunk@167690 bbb929c8-8fbe-4397-9dbb-9b2b20218538