ExcServerVariants test: use constructors to initialize test structures

util/mach/child_port_server_test.cc

Index: util/mach/child_port_server_test.cc
diff --git a/util/mach/child_port_server_test.cc b/util/mach/child_port_server_test.cc
index 05c22d7cbb32cbdb522ae544c85b1d1e0fb67477..ff9d83dde3f5637c66715d1d187d316550e4d35c 100644
--- a/util/mach/child_port_server_test.cc
+++ b/util/mach/child_port_server_test.cc
@@ -67,16 +67,12 @@ struct __attribute__((packed, aligned(4))) ChildPortCheckInRequest {
   child_port_token_t token;
 };
 
-struct __attribute__((packed, aligned(4))) MIGReply {
+struct MIGReply : public mig_reply_error_t {
   MIGReply() {
     memset(this, 0x5a, sizeof(*this));
     RetCode = KERN_FAILURE;
   }
 
-  mach_msg_header_t Head;
-  NDR_record_t NDR;
-  kern_return_t RetCode;
-
   void Verify() {
     EXPECT_EQ(implicit_cast<mach_msg_bits_t>(MACH_MSGH_BITS(0, 0)),
               Head.msgh_bits);