Add Copy Constructor for C# YogaNode

Summary:
Add Copy Constructor for convenience.

    YogaNode defaultStyle = new YogaNode
    {
      Width = 100,
      Height = 100,
    };

    YogaNode node0 = new YogaNode(defaultStyle)
    {
        FlexDirection = YogaFlexDirection.Row,
        JustifyContent = YogaJustify.FlexEnd,
    };

    YogaNode node1 = new YogaNode(defaultStyle)
    {
        AlignItems = YogaAlign.Center,
    };

Reviewed By: emilsjolander

Differential Revision: D4393234

fbshipit-source-id: b4b73071aab4dc1b3ae422969de659380bd0e3ad

csharp/Facebook.Yoga/YogaNode.cs

@@ -36,6 +36,12 @@ namespace Facebook.Yoga
             }
         }
 
+        public YogaNode(YogaNode srcNode)
+            : this()
+        {
+            CopyStyle(srcNode);
+        }
+
         public void Reset()
         {
             _measureFunction = null;

csharp/tests/Facebook.Yoga/YogaNodeTest.cs

@@ -276,6 +276,36 @@ namespace Facebook.Yoga
             Assert.AreEqual(100.Px(), node0.MaxHeight);
         }
 
+        [Test]
+        public void TestCopyConstructor()
+        {
+            YogaNode node0 = new YogaNode();
+            node0.MaxWidth = 80;
+
+            YogaNode node1 = new YogaNode(node0);
+            Assert.AreEqual(80.Px(), node1.MaxWidth);
+
+            YogaNode node2 = new YogaNode(node1)
+            {
+                MaxHeight = 90,
+            };
+            Assert.AreEqual(80.Px(), node2.MaxWidth);
+            Assert.AreEqual(90.Px(), node2.MaxHeight);
+
+            YogaNode node3 = new YogaNode(node0)
+            {
+                MaxWidth = 100,
+            };
+            Assert.AreEqual(100.Px(), node3.MaxWidth);
+
+            YogaNode node4 = new YogaNode(node2)
+            {
+                MaxWidth = 100,
+            };
+            Assert.AreEqual(100.Px(), node4.MaxWidth);
+            Assert.AreEqual(90.Px(), node4.MaxHeight);
+        }
+
         private void ForceGC()
         {
             GC.Collect(GC.MaxGeneration);