DevTools: Make the way inspectorFrontend is referenced in agents consistent in all of them.

R=yurys, pfeldman

Review URL: https://codereview.chromium.org/476153002

git-svn-id: svn://svn.chromium.org/blink/trunk@180381 bbb929c8-8fbe-4397-9dbb-9b2b20218538