Font JS not being attached on page manager pages

I've run into a strange issue where page manager pages don't have the needed advagg_font JavaScript attached.

I'm no expert by any means, but here's where I've narrowed the problem down to.

/**
 * Implements hook_page_build().
 */
function advagg_font_page_build(&$page) {
  // The $page variable is not an array; or the content key does not exist.
  if (!is_array($page) || !isset($page['content'])) {
    return;
  }

It seems even though $page['content'] exists later, it doesn't exist yet. No JavaScript gets attached so no font fun.

I see two possible solutions here:

1. Changing advagg_font_page_build() to advagg_font_page_alter() causes it to fire later in the process and appears to work ($page['content'] exists at that point), but I don't know enough to say if that's a viable solution.

2. Removing the check for $page['content'] also fixes the problem of course. Since nothing is read from $page and only appending to it, maybe that would work?

I'm not sure what direction would be best. If one of these seems feasible I'd be happy to roll a patch.