PasswordStoreFactory: use scoped_ptr (minor cleanup)

Get rid of manual lifetime handling of the login DB when creating a PasswordStore.

This is a minor clean-up issue. It used to be part of https://codereview.chromium.org/149393011/, but is now separated to make things clearer.

BUG=335155
R=gcasto@chromium.org

Review URL: https://codereview.chromium.org/153563003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248992 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248992