Update YogaConfig

Summary:
- Bugfix: Retain managed YogaConfig to prevent releasing unmanaged YogaConfig
- Use the same YogaConfig in the copy constructor
- Expose Set/GetUseWebDefaults APIs
- Split YogaConfig out from YogaNode.cs
Closes https://github.com/facebook/yoga/pull/496

Reviewed By: emilsjolander

Differential Revision: D4796178

Pulled By: splhack

fbshipit-source-id: cafabdc051ca914af547acbbf3d2246a5618e8bb

csharp/Facebook.Yoga/YogaConfig.cs

@@ -0,0 +1,67 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+using System;
+
+namespace Facebook.Yoga
+{
+    public class YogaConfig
+    {
+        private Native.YGConfigHandle _ygConfig;
+
+        public YogaConfig()
+        {
+            _ygConfig = Native.YGConfigNew();
+            if (_ygConfig.IsInvalid)
+            {
+                throw new InvalidOperationException("Failed to allocate native memory");
+            }
+        }
+
+        internal Native.YGConfigHandle Handle
+        {
+            get {
+                return _ygConfig;
+            }
+        }
+
+        public void SetExperimentalFeatureEnabled(
+            YogaExperimentalFeature feature,
+            bool enabled)
+        {
+            Native.YGConfigSetExperimentalFeatureEnabled(_ygConfig, feature, enabled);
+        }
+
+        public bool IsExperimentalFeatureEnabled(YogaExperimentalFeature feature)
+        {
+            return Native.YGConfigIsExperimentalFeatureEnabled(_ygConfig, feature);
+        }
+
+        public bool UseWebDefaults
+        {
+            get
+            {
+                return Native.YGConfigGetUseWebDefaults(_ygConfig);
+            }
+
+            set
+            {
+                Native.YGConfigSetUseWebDefaults(_ygConfig, value);
+            }
+        }
+
+        public float PointScaleFactor
+        {
+            set
+            {
+                Native.YGConfigSetPointScaleFactor(_ygConfig, value);
+            }
+        }
+    }
+}