[typescript] TypeScript-ify front_end/ui/ShortcutsScreen.js
[typescript] TypeScript-ify front_end/ui/ShortcutsScreen.js

Bug: 1011811
Change-Id: I09571be40f1572738e015b9911757a550dae1110
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2422947
Commit-Queue: Kim-Anh Tran <a72f38a79b5fac6811022b8a7fb84830d856c9c9@chromium.org>
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend