Replace HTMLConverter::_stringForNode by propertyValueForNode

Replace HTMLConverter::_stringForNode by propertyValueForNode
https://bugs.webkit.org/show_bug.cgi?id=130711

Reviewed by Sam Weinig.

Replaced all calls to HTMLConverter::_stringForNode by that to propertyValueForNode.

* editing/cocoa/HTMLConverter.mm:
(HTMLConverter::_computedAttributesForElement):
(HTMLConverter::_addAttachmentForElement):
(HTMLConverter::_enterElement):
(HTMLConverter::_addTableForElement):
(HTMLConverter::_addTableCellForElement):
(HTMLConverter::_processElement):
(HTMLConverter::_exitElement):
(HTMLConverter::_processText):
(HTMLConverter::_traverseNode):
(HTMLConverter::_traverseFooterNode):
(HTMLConverter::_stringForNode): Deleted.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@166273 268f45cc-cd09-0410-ab3c-d52691b4dbfc