DevTools: Show GPU utilization bar on timeline

The patch adds another strip below CPU utilization that
shows GPU process utilization.

BUG=298951

Review URL: https://codereview.chromium.org/46663010

git-svn-id: svn://svn.chromium.org/blink/trunk@161468 bbb929c8-8fbe-4397-9dbb-9b2b20218538