IDL compiler: sync Python to r165735

Python side to:
Remove V8HiddenPropertyName
https://codereview.chromium.org/135843008/

Also minor refactoring, now that code is simpler.

BUG=239771
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/148023002

git-svn-id: svn://svn.chromium.org/blink/trunk@165823 bbb929c8-8fbe-4397-9dbb-9b2b20218538