IDL compiler: Remove creationContext from v8SetReturnValue* in Python CG

Python sync to r160892:
We don't need to pass |creationContext| to v8SetReturnValue family
https://codereview.chromium.org/50073004/
(Nice simplification -- thanks haraken!)

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/51993002

git-svn-id: svn://svn.chromium.org/blink/trunk@160894 bbb929c8-8fbe-4397-9dbb-9b2b20218538