IDL compiler: sync Python to r163527

Python side to:
Generate a bit less code to handle failed arity checks.
https://codereview.chromium.org/105693002/

BUG=239771
NOTRY=true
TBR=haraken

Review URL: https://codereview.chromium.org/111793006

git-svn-id: svn://svn.chromium.org/blink/trunk@163666 bbb929c8-8fbe-4397-9dbb-9b2b20218538