IDL compiler: [ExposeJSAccessors] in Python

This adds the extended attribute to the Python CG.
Also:
* Fixes typo in Perl (missing ';')
* Refactors Python slightly to use {% set %} instead of inline {% if %}
* Removes dead code in Python (|has_runtime_enabled_attributes|):
  no longer needed since always adding includes for IDL type.

Python side to:
Implement [ExposeJSAccessors] IDL extended attribute
https://codereview.chromium.org/73173003/

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/70093004

git-svn-id: svn://svn.chromium.org/blink/trunk@162181 bbb929c8-8fbe-4397-9dbb-9b2b20218538