DevTools: Fix default Sources panel editor selection

No editor is currently selected in the Sources panel by default. Regressed in r166357.

R=vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/153633003

git-svn-id: svn://svn.chromium.org/blink/trunk@166388 bbb929c8-8fbe-4397-9dbb-9b2b20218538