DevTools: [JsDocValidator] Handle types declared as top-level vars

This patch allows the validator checks to handle types declared as
/** @constructor */ var MyType = function() {}

R=pfeldman, sergeyv

Review URL: https://codereview.chromium.org/218993012

git-svn-id: svn://svn.chromium.org/blink/trunk@170555 bbb929c8-8fbe-4397-9dbb-9b2b20218538