webapi: don't upgrade custom elements in a windowless document

A custom element created in a windowless document (think DOMParser, or new
Document(), ...) doesn't have a custom element registry and thus should remain
undefined.

This repurposes the existing Frame._skip_custom_element_upgrade boolean into a
tri-state enum to capture the 3 possible states: `construct` and `bare_context`
capture the previous boolean state, and `undefined` is now used for this third
state.

This fixes (non-fatal) errors on decathlon.com. It also fixes a few WPT cases
and advances a few more (which are now failing for a different reason).