Support idle time on Flame Chart

We already show aggregated idle time as (idle) node in the grid view. This CL adds support for idle time on the flame chart.

BUG=268947
R=alph@chromium.org

Review URL: https://codereview.chromium.org/22417003

git-svn-id: svn://svn.chromium.org/blink/trunk@156095 bbb929c8-8fbe-4397-9dbb-9b2b20218538