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

java/com/facebook/csslayout/CSSExperimentalFeature.java

@@ -10,7 +10,8 @@
 package com.facebook.csslayout;
 
 public enum CSSExperimentalFeature {
-__EMPTY(-1);
+  ROUNDING(0);
+
   private int mIntValue;
 
   CSSExperimentalFeature(int intValue) {
@@ -23,6 +24,7 @@ __EMPTY(-1);
 
   public static CSSExperimentalFeature fromInt(int value) {
     switch (value) {
+      case 0: return ROUNDING;
       default: throw new IllegalArgumentException("Unkown enum value: " + value);
     }
   }