Disallow explicit scoped_ptr<T> construction from NULL.

This matches the behavior of std::unique_ptr<T>, which is the
long-term replacement for scoped_ptr<T>.

BUG=173286
R=jyasskin@chromium.org, mark@chromium.org

Review URL: https://codereview.chromium.org/17028014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@208773 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed