IDL: Add Function and VoidFunction as built-in callback function types

They are included as "common definitions" in WebIDL and can thus be used
by other specifications without definition.

Review URL: https://codereview.chromium.org/715233003

git-svn-id: svn://svn.chromium.org/blink/trunk@185216 bbb929c8-8fbe-4397-9dbb-9b2b20218538