FlameChart: speed up bars drawing procedure.

It used to switch fillStyle for every bar. (N times)
With new implementation it will switch fillStyle K times
where K is the number of functions by drawing all the bars for one color in a sequence.

BUG=none
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/23533037

git-svn-id: svn://svn.chromium.org/blink/trunk@157451 bbb929c8-8fbe-4397-9dbb-9b2b20218538