Remove m_securityOrigin from XMLHttpRequest
https://bugs.webkit.org/show_bug.cgi?id=115881

Reviewed by Alexey Proskuryakov.

The security origin passed to XMLHttpRequest::create was always null, so just get rid of the member variable completely.

* xml/XMLHttpRequest.cpp:
(WebCore::XMLHttpRequest::create):
(WebCore::XMLHttpRequest::XMLHttpRequest):
(WebCore::XMLHttpRequest::securityOrigin):
* xml/XMLHttpRequest.h:
(XMLHttpRequest):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149853 268f45cc-cd09-0410-ab3c-d52691b4dbfc