DevTools: drop home-grown Map implementation in favor of native ES6 Map

R=pfeldman, vsevik, apavlov

Review URL: https://codereview.chromium.org/674683002

git-svn-id: svn://svn.chromium.org/blink/trunk@184501 bbb929c8-8fbe-4397-9dbb-9b2b20218538