Fix wrong `this` to access a static variable in a static method
Fix wrong `this` to access a static variable in a static method

This bug caused unexpected result on tests:

- test/files/repeating_elements/priority_of_dropping.html
- test/files/nth-fragment/table.html
- test/files/after_if_continues/after-if-continues{,_vertical}.html

vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js