Crash due to incorrect firing of mutation event during class attribute parsing.

Crash due to incorrect firing of mutation event during class attribute parsing.
https://bugs.webkit.org/show_bug.cgi?id=78537

Reviewed by Ryosuke Niwa.

Source/WebCore:

Test: fast/dom/class-attr-change-double-mutation-fire.html

* dom/StyledElement.cpp:
(WebCore::StyledElement::classAttributeChanged):

LayoutTests:

* fast/dom/class-attr-change-double-mutation-fire-expected.txt: Added.
* fast/dom/class-attr-change-double-mutation-fire.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107726 268f45cc-cd09-0410-ab3c-d52691b4dbfc