DevTools: Use chrome's style buttons on panelEnablerView instead of oval's buttons

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/442453002

git-svn-id: svn://svn.chromium.org/blink/trunk@179602 bbb929c8-8fbe-4397-9dbb-9b2b20218538