FlameChart: convert drawOverview method into static function for reusing in TimelinePanel

_calculateDrawData and _drawOverviewCanvas were made static.

A small portion of the function was extracted into resetCanvas method.

BUG=319999
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/88023002

git-svn-id: svn://svn.chromium.org/blink/trunk@162688 bbb929c8-8fbe-4397-9dbb-9b2b20218538