Now ignores obsolete sync nodes without visit transitions.

Also removed assertion that was erroneously triggered by obsolete sync nodes.

BUG=none
TEST=run chrome against a database that contains obsolete typed url sync nodes.

Review URL: http://codereview.chromium.org/7129069

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88846 0039d316-1c4b-4281-b951-d872f2087c98