Revert 161595 "Update the accessibility tree when a modal dialog..."

Causing use-after-frees on ClusterFuzz.

> Update the accessibility tree when a modal dialog is opened/closed.
> 
> When a node's inertness changes, we must notify the AX tree since
> the node may need to be added or removed from the tree.
> 
> This patch does the following.
> 1) When an modal dialog opens/closes, essentially we traverse the AX
> tree and call notifyIfIgnoredValueChanged on each object. However, once
> an object is found whose ignored value changed, it's not necessary to
> descend into its subtree by virtue of the childrenCleared/addChild
> mechanism.
> 
> An earlier attempt called notifyIfIgnoredValueChanged only on immediate
> children of the HTMLBodyElement. This is insufficient because siblings
> of <dialog> are roots of newly (or formerly) inert subtrees, and some
> objects are not descendants of body due to renderer reparenting (e.g.
> modal dialogs).
> 
> 2) Modifies AXObject::isInertOrAriaHidden to climb the AX tree until an
> object with non-null node() is found, when evaluating inertness. This is
> necessary because some objects such as one created with
> MenuListPopupRole don't have a node, but should be considered inert.
> 
> The test for this patch is Chrome-side:
> https://codereview.chromium.org/64273003
> 
> BUG=304779
> 
> Review URL: https://codereview.chromium.org/59663005

TBR=falken@chromium.org

Review URL: https://codereview.chromium.org/68003002

git-svn-id: svn://svn.chromium.org/blink/trunk@161694 bbb929c8-8fbe-4397-9dbb-9b2b20218538