Fix crash when a focused node is removed while in processing focusin event.

Fix crash when a focused node is removed while in processing focusin event.
https://bugs.webkit.org/show_bug.cgi?id=76656

Reviewed by Dimitri Glazkov.

Source/WebCore:

Test: fast/events/focus-remove-focuesed-node.html

* dom/Document.cpp:
(WebCore::Document::setFocusedNode):

LayoutTests:

* fast/events/focus-remove-focuesed-node-expected.txt: Added.
* fast/events/focus-remove-focuesed-node.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105665 268f45cc-cd09-0410-ab3c-d52691b4dbfc