DevTools: recognize clicks on Timeline's category label in status bar

We used to only honor checkbox clicks, now we also process label clicks as well.
Drive-by: use createChild() to build DOM.

BUG=
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/23880003

git-svn-id: svn://svn.chromium.org/blink/trunk@157299 bbb929c8-8fbe-4397-9dbb-9b2b20218538