DevTools: [JsDoc] Annotate @return types in profiler-related code

R=aandrey, eustas, loislo, yurys

Review URL: https://codereview.chromium.org/143323011

git-svn-id: svn://svn.chromium.org/blink/trunk@165631 bbb929c8-8fbe-4397-9dbb-9b2b20218538