libaddressinput: less copying of rules data

- Change Downloader interface to pass scoped_ptr
- Pass ownership of string through Timestamp/Checksum wrapping and unwrapping (also make it append instead of prepend)

BUG=337679

Review URL: https://codereview.chromium.org/145553010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246969 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246969