Update third_party/protobuf to protobuf-v3.0.0-beta-3

third_party/protobuf/csharp/src/Google.Protobuf/ProtoPreconditions.cs

Index: third_party/protobuf/csharp/src/Google.Protobuf/ProtoPreconditions.cs
diff --git a/third_party/protobuf/csharp/src/Google.Protobuf/Preconditions.cs b/third_party/protobuf/csharp/src/Google.Protobuf/ProtoPreconditions.cs
similarity index 89%
rename from third_party/protobuf/csharp/src/Google.Protobuf/Preconditions.cs
rename to third_party/protobuf/csharp/src/Google.Protobuf/ProtoPreconditions.cs
index 2db35ff63a730e9a70f3dcd91f98d0d195b61ea2..abaeb9b481d4053201dff005098f4f8e6038abb7 100644
--- a/third_party/protobuf/csharp/src/Google.Protobuf/Preconditions.cs
+++ b/third_party/protobuf/csharp/src/Google.Protobuf/ProtoPreconditions.cs
@@ -35,9 +35,14 @@ using System;
 namespace Google.Protobuf
 {
     /// <summary>
-    /// Helper methods for throwing exceptions
+    /// Helper methods for throwing exceptions when preconditions are not met.
     /// </summary>
-    public static class Preconditions
+    /// <remarks>
+    /// This class is used internally and by generated code; it is not particularly
+    /// expected to be used from application code, although nothing prevents it
+    /// from being used that way.
+    /// </remarks>
+    public static class ProtoPreconditions
     {
         /// <summary>
         /// Throws an ArgumentNullException if the given value is null, otherwise