IDL compiler: sync Python to r163234

Python side to:
Manage FileSystem callbacks with OwnPtr instead of refcounting
https://codereview.chromium.org/105523002/

This removes the legacy ref count special case for callback
interfaces, so the Python CG is also fully updated.

BUG=323681
NOTRY=true
TBR=haraken

Review URL: https://codereview.chromium.org/107593002

git-svn-id: svn://svn.chromium.org/blink/trunk@163296 bbb929c8-8fbe-4397-9dbb-9b2b20218538