Convert safe-browsing SBPrefix from int32 to uint32.

Because int32 for something mostly used for bitwise operations is silly.

PrefixSet serialized sorted SBPrefix values.  Version the database and add code
to re-order things on read.

BUG=346405

Review URL: https://codereview.chromium.org/178593002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253924 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253924