[GTK] editing/inserting/4960120-2.html flaky crash

[GTK] editing/inserting/4960120-2.html flaky crash
https://bugs.webkit.org/show_bug.cgi?id=76815

Reviewed by Martin Robinson.

Source/WebCore:

Check if the node for the first parent object not ignoring
accessibility is null before using it. This might happen with
certain kind of accessibility objects, such as the root one (the
scroller containing the webArea object as its only child).

* accessibility/gtk/WebKitAccessibleWrapperAtk.cpp:
(objectFocusedAndCaretOffsetUnignored): Add missing null check.

LayoutTests:

* platform/gtk/test_expectations.txt: Unskipping tests that
shouldn't be crashing from now on.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106472 268f45cc-cd09-0410-ab3c-d52691b4dbfc