FlameChart: implement highlighted element with help of div.

As a result we can use single collor instead of pair.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/183483003

git-svn-id: svn://svn.chromium.org/blink/trunk@168084 bbb929c8-8fbe-4397-9dbb-9b2b20218538