DevTools: Correctly track current focus element when focus is lost (gone to document.body)

We use WebInspector.currentFocusElemen() getter to check whether given element has focus and these checks fail when focus is lost (e.g. clicking on console toolbar breaks ctrl-tilde shortcut).
R=pfeldman

Review URL: https://codereview.chromium.org/177793002

git-svn-id: svn://svn.chromium.org/blink/trunk@167748 bbb929c8-8fbe-4397-9dbb-9b2b20218538