DevTools: [JsDoc] Make Array item types strictly nullable or non-nullable

R=aandrey, eustas.ru, vsevik, yurys
BUG=324831

Review URL: https://codereview.chromium.org/104093002

git-svn-id: svn://svn.chromium.org/blink/trunk@163171 bbb929c8-8fbe-4397-9dbb-9b2b20218538