IDL compiler: [PerWorldBindings] methods :: basic

This is a first CL for [PerWorldBindings].
This is one of the most complex extended attributes,
so I'll break it up over a few CLs.

Also added some {% set %} statements to the templates to replace
inline {% if %}...{% else %}...{% endif %} blocks, which are illegible,
but still keep the logic in the template.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/63633003

git-svn-id: svn://svn.chromium.org/blink/trunk@161510 bbb929c8-8fbe-4397-9dbb-9b2b20218538