Add message sizing to basic IPC traits and struct macros.

mojo/public/cpp/bindings/lib/native_serialization.h

Index: mojo/public/cpp/bindings/lib/native_serialization.h
diff --git a/mojo/public/cpp/bindings/lib/native_serialization.h b/mojo/public/cpp/bindings/lib/native_serialization.h
index 3c2e54efac58d2416d45b6ca3934afd582f94cdd..2d16ac139325d30606887ac17f0c6adf6d4b6e69 100644
--- a/mojo/public/cpp/bindings/lib/native_serialization.h
+++ b/mojo/public/cpp/bindings/lib/native_serialization.h
@@ -29,7 +29,9 @@ struct ShouldUseNativeSerializer { static const bool value = false; };
 
 template <typename T>
 size_t GetSerializedSizeNative_(const T& value) {
-  return IPC::ParamTraits<T>::GetSize(value);
+  base::PickleSizer sizer;
+  IPC::ParamTraits<T>::GetSize(&sizer, value);
+  return sizer.payload_size();
 }
 
 template <typename T>
@@ -46,8 +48,20 @@ void SerializeNative_(const T& value,
   base::Pickle* pickle = pickler->pickle();
   const char* data_start = pickle->end_of_payload();
 
+#if !defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)
+  size_t size_before_write = pickle->payload_size();
+#endif
+
   IPC::ParamTraits<T>::Write(pickle, value);
 
+#if !defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)
+  DCHECK_GE(pickle->payload_size(), size_before_write);

[jam, 2016/02/04 18:34:01]

this is checking that the payload size didn't shrink? how could that ever
happen?

+  // Explicitly validate that the value returned by GetSize() always equals the
+  // number of bytes actually written by Write().
+  size_t bytes_written = pickle->payload_size() - size_before_write;
+  DCHECK_EQ(bytes_written, GetSerializedSizeNative_(value));
+#endif
+
   // Fix up the ArrayHeader so that num_elements contains the length of the
   // pickled data.
   size_t pickled_size = pickle->end_of_payload() - data_start;