PDFPlugin form editing depends on having a Document that has the same size and position as the plugin; this is true not only in the full-main-frame case, but also in the <iframe> case. Therefore, we can enable form editing for <iframe> PDFPlugins.
<rdar://problem/12752339>
Actually, <rdar://problem/10326917>.
Created attachment 176075 [details] patch
This depends on https://bugs.webkit.org/show_bug.cgi?id=103286.
Thanks, Dan! http://trac.webkit.org/changeset/136317