Remove category getter from TimelineModel.Record

Also moved recordStyle method to WebInspector.TimelineUIUtilsImpl as it is supposed to work with records build on Timeline agent messages and doesn't support trace events.

BUG=361045
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/350693002

git-svn-id: svn://svn.chromium.org/blink/trunk@176734 bbb929c8-8fbe-4397-9dbb-9b2b20218538