Rewrite WebHTMLConverter::_elementIsBlockLevel in C++

Rewrite WebHTMLConverter::_elementIsBlockLevel in C++
https://bugs.webkit.org/show_bug.cgi?id=130287

Reviewed by Andreas Kling.

Rewrote _elementIsBlockLevel as HTMLConverterCaches::isBlockElement. Also removed the code to update
_elementIsBlockLevel in _traverseNode as computing the value of _elementIsBlockLevel is now fast.

* platform/mac/HTMLConverter.h:
* platform/mac/HTMLConverter.mm:
(HTMLConverterCaches::isBlockElement):
(-[WebHTMLConverter _elementIsBlockLevel:]):
(-[WebHTMLConverter _traverseNode:depth:embedded:]):
(-[WebHTMLConverter dealloc]):
(-[WebHTMLConverter init]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165787 268f45cc-cd09-0410-ab3c-d52691b4dbfc