DevTools: Extract settings-related UI construction methods into SettingsUI (ui)

R=aandrey, eustas, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/144803002

git-svn-id: svn://svn.chromium.org/blink/trunk@165523 bbb929c8-8fbe-4397-9dbb-9b2b20218538