DevTools: [JsDoc] Make sure __proto__ is assigned properly for types having @extends

R=caseq, eustas, vsevik, yurys

Review URL: https://codereview.chromium.org/116083004

git-svn-id: svn://svn.chromium.org/blink/trunk@164334 bbb929c8-8fbe-4397-9dbb-9b2b20218538