Added feature to use rounded values

Summary:
Added an experimental feature to allow to use only rounded values. See #184. It's not a perfect solution and definitely  can be further improved. I'm looking forward to your ideas.
Closes https://github.com/facebook/css-layout/pull/256

Reviewed By: splhack

Differential Revision: D4214168

Pulled By: emilsjolander

fbshipit-source-id: 6293352d479b7b4dad258eb3f9e0afaa11cf7236

CSSLayout/CSSEnums.h

@@ -67,6 +67,7 @@ typedef enum CSSDirection {
 } CSSDirection;
 
 typedef enum CSSExperimentalFeature {
+  CSSExperimentalFeatureRounding,
   CSSExperimentalFeatureCount,
 } CSSExperimentalFeature;