The page-name property only applies to boxes which can create class A
breakpoints. One of the qualifiers for this is a block-level item, which is
the child of a block frame. Additionally, page-name propagation only occurs
through frames for which the page-name property applies, because where it does
not apply the auto page-name is propagated instead.

This means we only need to worry about block frames, and only use specified
page-name for BlockOutside frames.

Differential Revision: https://phabricator.services.mozilla.com/D155285