views: Make View::set_border() take a scoped_ptr<>.

This makes the ownership situation much clearer. It also renames the function to SetBorder() (since it is not a simple accessor and can free previous instances of Border), and creates a Border::NullBorder() method (since "SetBorder(scoped_ptr<>())" is not clear).

BUG=none
R=sky@chromium.org

Review URL: https://codereview.chromium.org/145033006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246990 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246990