Add virtual destructors for the default implementations as well.

src/cpp/client/client_context.cc

@@ -48,6 +48,7 @@ namespace grpc {
 class DefaultGlobalClientCallbacks GRPC_FINAL
     : public ClientContext::GlobalCallbacks {
  public:
+  ~DefaultGlobalClientCallbacks GRPC_OVERRIDE {}
   void DefaultConstructor(ClientContext* context) GRPC_OVERRIDE {}
   void Destructor(ClientContext* context) GRPC_OVERRIDE {}
 };

src/cpp/server/server.cc

@@ -53,6 +53,7 @@ namespace grpc {
 
 class DefaultGlobalCallbacks GRPC_FINAL : public Server::GlobalCallbacks {
  public:
+  ~DefaultGlobalCallbacks GRPC_OVERRIDE {}
   void PreSynchronousRequest(ServerContext* context) GRPC_OVERRIDE {}
   void PostSynchronousRequest(ServerContext* context) GRPC_OVERRIDE {}
 };