DevTools: Handle the F8 shortcut forwarded from the inspected page

R=pfeldman, vsevik
BUG=160510

Review URL: https://codereview.chromium.org/225873004

git-svn-id: svn://svn.chromium.org/blink/trunk@171463 bbb929c8-8fbe-4397-9dbb-9b2b20218538