DevTools: FlameChart: minor cleanup on the way to good draw performance.

maxStackDepth calculation that we can do once was moved out from repeating action.

BUG=302913
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/25688002

git-svn-id: svn://svn.chromium.org/blink/trunk@158700 bbb929c8-8fbe-4397-9dbb-9b2b20218538