Covert self time to milliseconds before passing it into pie chart generator

BUG=361045
TBR=alph@chromium.org

Review URL: https://codereview.chromium.org/307093002

git-svn-id: svn://svn.chromium.org/blink/trunk@175132 bbb929c8-8fbe-4397-9dbb-9b2b20218538