IDL compiler: sync Python to r167547

Python side to:
Pass implementation object to supplemental classes by reference
https://codereview.chromium.org/171333003

BUG=239771
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/176823002

git-svn-id: svn://svn.chromium.org/blink/trunk@167652 bbb929c8-8fbe-4397-9dbb-9b2b20218538