Add implicit nullptr constructor to WebPrivateOwnPtr.
Add implicit nullptr constructor to WebPrivateOwnPtr.

Make WebPrivateOwnPtr constructible from nullptr implicitly.
This is a preparation for WebCallbacks refactoring.

BUG=493531

Review URL: https://codereview.chromium.org/1237773002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@198847 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk