DevTools: Add support for adding folders to workspace with drag and drop (renderer part)

browser part: https://chromiumcodereview.appspot.com/57073004/

BUG=314476
R=pfeldman, kinuko

Review URL: https://codereview.chromium.org/57373002

git-svn-id: svn://svn.chromium.org/blink/trunk@161423 bbb929c8-8fbe-4397-9dbb-9b2b20218538