DevTools: Remove a workaround for missing native Set in Context.js

R=vsevik@chromium.org, vsevik

Review URL: https://codereview.chromium.org/695443002

git-svn-id: svn://svn.chromium.org/blink/trunk@184712 bbb929c8-8fbe-4397-9dbb-9b2b20218538