IDL compiler: Move includes to global variable

This refactoring moves includes to a global variable
(use a module to implement Singleton pattern in Python),
which simplifies function calls substantially.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/38643004

git-svn-id: svn://svn.chromium.org/blink/trunk@160432 bbb929c8-8fbe-4397-9dbb-9b2b20218538