Hide WebContentsImpl::AddCreatedCallback so that no more users leak in.

BUG=277617
R=avi@chromium.org

Review URL: https://codereview.chromium.org/68673003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234570 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234570