fast/encoding/parser-tests-*.html tests sometimes crash
https://bugs.webkit.org/show_bug.cgi?id=108058

Reviewed by Chris Fleizach.

Source/WebCore:

To avoid calling accessibilityIsIgnored while the render
tree is unstable, call accessibilityIsIgnored in the
notification timer handler, only for childrenChanged
notifications.

This exposed a problem where notifications queued on
objects can fire after the object has been deleted; fix that
by checking the object's id, which is always set to 0 when
removed from the tree.

Covered by existing tests.

* accessibility/AXObjectCache.cpp:
(WebCore::AXObjectCache::childrenChanged):
(WebCore::AXObjectCache::notificationPostTimerFired):

LayoutTests:

Make test less brittle by (1) giving the iframe an aria-role so
it's never ignored, and (2) using accessibilityElementById instead
of assuming an element is in a specific place in the AX tree.

* accessibility/loading-iframe-updates-axtree.html:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142385 268f45cc-cd09-0410-ab3c-d52691b4dbfc