Rewrite WebHTMLConverter::_getComputedFloat in C++

Rewrite WebHTMLConverter::_getComputedFloat in C++
https://bugs.webkit.org/show_bug.cgi?id=130284

Reviewed by Andreas Kling.

Rewrote _getComputedFloat as HTMLConverterCaches::floatPropertyValueForNode.

* platform/mac/HTMLConverter.h:
* platform/mac/HTMLConverter.mm:
(HTMLConverterCaches::floatPropertyValueForNode):
(-[WebHTMLConverter _getFloat:forNode:property:]): Now that computing the float value is fast, we don't need to
store it in the cache.
(-[WebHTMLConverter dealloc]):
(-[WebHTMLConverter init]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165786 268f45cc-cd09-0410-ab3c-d52691b4dbfc