adjust update check to occur once per day
record date of last update check in preference
only check for update once per day
add plugin for testing update functionality
remove unused class
remove UUID added in https://codereview.chromium.org/642393002/

BUG=
R=lukechurch@google.com, pquitslund@google.com

Review URL: https://codereview.chromium.org//685033005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41511 260f80e4-7a28-3924-810f-c04153c831b5