DevTools: show background paint events on tracing based Timeline waterfall

The events are displayed as hollow rectangles like the were on the old Timeline.

BUG=414243

Review URL: https://codereview.chromium.org/619853004

git-svn-id: svn://svn.chromium.org/blink/trunk@183124 bbb929c8-8fbe-4397-9dbb-9b2b20218538