DevTools: [JsDocValidator] Add check that requires @return annotation for all API methods that return value

An API method is a class method whose name does not start with an underscore ('_').

R=pfeldman, sergeyv
NOTRY=true

Review URL: https://codereview.chromium.org/143003015

git-svn-id: svn://svn.chromium.org/blink/trunk@165644 bbb929c8-8fbe-4397-9dbb-9b2b20218538