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

third_party/protobuf/csharp/protos/unittest_issues.proto

 syntax = "proto3";
 
 // These proto descriptors have at one time been reported as an issue or defect.
 // They are kept here to replicate the issue, and continue to verify the fix.
 
 // Issue: Non-"Google.Protobuffers" namespace will ensure that protobuffer library types are qualified
 option csharp_namespace = "UnitTest.Issues.TestProtos";
 
 package unittest_issues;
 option optimize_for = SPEED;
@@ skipping 98 matching lines @@
     int32 o1_int32 = 5;
   }
   
   string plain_string = 1;
   
   oneof o2 {
     int32 o2_int32 = 6;
     string o2_string = 3;
   }
   
-}
+}
+
+message TestJsonName {
+  // Message for testing the effects for of the json_name option
+  string name = 1;
+  string description = 2 [json_name = "desc"];
+  string guid = 3 [json_name = "exid"];
+}