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

This patch also:
- kills StringSet as its no more needed
- removes inspector/map.html

R=vsevik, apavlov, pfeldman

Review URL: https://codereview.chromium.org/671963003

git-svn-id: svn://svn.chromium.org/blink/trunk@184569 bbb929c8-8fbe-4397-9dbb-9b2b20218538