[Mac] Rewrite WebHTMLConverter::_computedStringForNode in C++

[Mac] Rewrite WebHTMLConverter::_computedStringForNode in C++
https://bugs.webkit.org/show_bug.cgi?id=130274

Reviewed by Andreas Kling.

Rewrote WebHTMLConverter::_computedStringForNode as HTMLConverterCaches::propertyValueForNode.

* platform/mac/HTMLConverter.h: Removed unused caches.
* platform/mac/HTMLConverter.mm:
(HTMLConverterCaches::computedStylePropertyForElement): Added use const String& instead of String& on property name.
(HTMLConverterCaches::inlineStylePropertyForElement): Ditto.
(HTMLConverterCaches::propertyValueForNode): Renamed and rewritten from _computedStringForNode.
(-[WebHTMLConverter _stringForNode:property:]): Simply return the computed value instead of storing it in the cache
now that the computing the value is fast.
(-[WebHTMLConverter dealloc]):
(-[WebHTMLConverter init]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165761 268f45cc-cd09-0410-ab3c-d52691b4dbfc