IDL compiler: callback functions

Simple case, 2 real uses (Document.idl and Promise.idl).
Main content is that both |any| and callback functions are treated
internally as ScriptValue, so putting this in preprocess_idl_type
and updating dictionaries largely does it.

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/105993003

git-svn-id: svn://svn.chromium.org/blink/trunk@164166 bbb929c8-8fbe-4397-9dbb-9b2b20218538