little refactoring for Timespec

src/csharp/GrpcCore/Internal/Timespec.cs

@@ -46,6 +46,14 @@ namespace Google.GRPC.Core.Internal
             }
         }
 
+        internal static int NativeSize
+        {
+            get
+            {
+                return gprsharp_sizeof_timespec();
+            }
+        }
+
         /// <summary>
         /// Creates a GPR deadline from current instant and given timeout.
         /// </summary>

src/csharp/GrpcCoreTests/TimespecTest.cs

 using System;
 using NUnit.Framework;
+using System.Runtime.InteropServices;
 using Google.GRPC.Core.Internal;
 
 namespace Google.GRPC.Core.Internal.Tests
@@ -12,6 +13,18 @@ namespace Google.GRPC.Core.Internal.Tests
             var timespec = Timespec.Now;
         }
 
+        [Test]
+        public void InfFuture()
+        {
+            var timespec = Timespec.InfFuture;
+        }
+
+        [Test]
+        public void TimespecSizeIsNativeSize()
+        {
+            Assert.AreEqual(Timespec.NativeSize, Marshal.SizeOf(typeof(Timespec)));
+        }
+
         [Test]
         public void Add()
         {