<http://webkit.org/b/107809>

Reviewed by Antti Koivisto.

Add an isValid() method to CSSSelectorList and use that where applicable instead of checking
if first() is a null pointer.

* css/CSSPageRule.cpp:
(WebCore::CSSPageRule::setSelectorText):
* css/CSSSelectorList.cpp:
* css/CSSSelectorList.h:
(WebCore::CSSSelectorList::isValid):
(WebCore::CSSSelectorList::first):
(CSSSelectorList):
* css/CSSStyleRule.cpp:
(WebCore::CSSStyleRule::setSelectorText):
* html/shadow/HTMLContentElement.cpp:
(WebCore::HTMLContentElement::validateSelect):
* inspector/InspectorStyleSheet.cpp:
(WebCore::checkStyleRuleSelector):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140677 268f45cc-cd09-0410-ab3c-d52691b4dbfc