[Devtools] Remove workaround for ignored nodes being included in accessibility tree

https://crrev.com/c/1839220 added a workaround for a bug where ignored
nodes in the accessibility tree were causing the partial AX tree to
break, because we weren't consistent about only using unignored nodes.

https://crrev.com/c/2505362 fixed the underlying issue, so this
workaround isn't necessary any more.

Removing that hack revealed some other issues, also fixed here:
- When a Node had no associated AXObject (typically because it has no
  LayoutObject), it wasn't correctly added as a child of its
  unignored parent
- The logic for adding children of children of an ignored node when the
  direct child has no associated Node (i.e. <select> MenuItemPopup) was
  buggy.

Bug: 887173,1011245
Change-Id: I0a6015ae6bd1b3597ec83c08993d3c4f8e36a850
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2812662


Reviewed-by: Meredith Lane <meredithl@chromium.org>
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#871850}