DevTools: [JSDocValidator] Require enum keys to start with an uppercase letter

R=sergeyv
NOTRY=true

Review URL: https://codereview.chromium.org/417133002

git-svn-id: svn://svn.chromium.org/blink/trunk@178926 bbb929c8-8fbe-4397-9dbb-9b2b20218538