Bug https://bugs.webkit.org/show_bug.cgi?id=49106 will include render widget attached children in the main accessibility tree for all ports expect for mac. The mac port currently forwards methods directly the render widget attachment view. Perhaps the forwarding can be removed in favor of including the render widget attachment in the main accessibility tree.
Closing b/c this was initiated for Chromium which has now forked. If you want this revisited for WebKit, re-open with justification.