IDL compiler: sync Python to r163455

Python side to:
Slim down generated Callback classes and enforce more at compile-time
https://codereview.chromium.org/107343004/

TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/99853004

git-svn-id: svn://svn.chromium.org/blink/trunk@163483 bbb929c8-8fbe-4397-9dbb-9b2b20218538