DevTools: polish timeline flame chart: fix viewport height, render grid with background, use overlay scroll on mac, paint ruler background.

R=alph@chromium.org

Review URL: https://codereview.chromium.org/252593002

git-svn-id: svn://svn.chromium.org/blink/trunk@172611 bbb929c8-8fbe-4397-9dbb-9b2b20218538