DevTools: fix primary selection in multiple selections created with ctrl-d

This patch sets correct index to the multiple selections created with
ctrl-d.

Review URL: https://codereview.chromium.org/220203007

git-svn-id: svn://svn.chromium.org/blink/trunk@170643 bbb929c8-8fbe-4397-9dbb-9b2b20218538