AX: Hidden aria table crash
https://bugs.webkit.org/show_bug.cgi?id=139856

Reviewed by Chris Fleizach.

Source/WebCore:

Change axCaption to pointer and check it's value because
AXObjectCache::getOrCreate() can return with nullptr.

Test: accessibility/aria-hidden-crash.html

* accessibility/AccessibilityTable.cpp:
(WebCore::AccessibilityTable::addChildren):

LayoutTests:

Add layout test to cover this crash.

* accessibility/aria-hidden-crash-expected.txt: Added.
* accessibility/aria-hidden-crash.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@177627 268f45cc-cd09-0410-ab3c-d52691b4dbfc