DevTools: Simplify _assignParentsInProfile

Do not create extra objects while traversing.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/131103004

git-svn-id: svn://svn.chromium.org/blink/trunk@165530 bbb929c8-8fbe-4397-9dbb-9b2b20218538