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.

Also revert some reset() uses that were introduced while waiting for
the same nullptr change to land.

R=abarth@chromium.org
BUG=

Review URL: https://codereview.chromium.org/173713008

git-svn-id: svn://svn.chromium.org/blink/trunk@167612 bbb929c8-8fbe-4397-9dbb-9b2b20218538