Since we now only enable annotation of iframes that are explicitly opted
in by the containing page, we can avoid checking the size of the iframe
as well. That check was originally added to filter out ads and hidden
iframes.

This resolves an issue with Epub.js where an iframe initially has zero
width. This is because book content is laid out into a variable number
of fixed-width columns and the iframe's width is set to the total width
of all columns. Until the content is loaded, the number of columns and
hence the width, is zero.