DevTools: Fix function return types in the frontend (except the heap snapshot-related code)

R=aandrey, eustas, vsevik

Review URL: https://codereview.chromium.org/120033005

git-svn-id: svn://svn.chromium.org/blink/trunk@164377 bbb929c8-8fbe-4397-9dbb-9b2b20218538