FlameChart: convert colors magic into dataProvider.entryColor getter

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/185533004

git-svn-id: svn://svn.chromium.org/blink/trunk@168361 bbb929c8-8fbe-4397-9dbb-9b2b20218538