DevTools: [JsDoc] Make generated protocol type @typedefs strictly non-nullable

RDP does not allow the transfer of null objects, hence the respective @typedefs
should not allow nulls.

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

Review URL: https://codereview.chromium.org/100363004

git-svn-id: svn://svn.chromium.org/blink/trunk@163180 bbb929c8-8fbe-4397-9dbb-9b2b20218538