Change scrollbars to bind to workspaces instead of random DOM elements.  Move get/setMetrics from scrollbars to workspaces.  No functional changes.  Also fixes invisible horizontal scrollbar issue reported by Spencer Gordon.

R=spertus@google.com

Review URL: https://codereview.appspot.com/13315043

git-svn-id: http://blockly.googlecode.com/svn/trunk@1293 c400ca83-b69d-9dd7-9705-49c6b8615e23