DevTools: [JsDocValidator] Avoid duplicates in the checked files list

Now that we have standalone modules, certain files can encounter in more
that one module.

R=vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/137833003

git-svn-id: svn://svn.chromium.org/blink/trunk@165936 bbb929c8-8fbe-4397-9dbb-9b2b20218538