TimelineFlameChart: draw parent bar with different color if there is a child on top of it.

BUG=348993
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/184313003

git-svn-id: svn://svn.chromium.org/blink/trunk@168447 bbb929c8-8fbe-4397-9dbb-9b2b20218538