IDL (Python): allow optional values to be undefined in overload resolution

Python side to:
IDL: allow optional values to be undefined in overload resolution
https://codereview.chromium.org/152413005/

BUG=340458
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/153743002

git-svn-id: svn://svn.chromium.org/blink/trunk@166383 bbb929c8-8fbe-4397-9dbb-9b2b20218538