Move size constant out into message_bridge_protocol.h

Sarah points out I really should make hard-coded numbers into constants.
She's not wrong.  Done.

This was done in a follow-on review to merge the fix faster.

Signed-off-by: Austin Schuh <austin.schuh@bluerivertech.com>
Change-Id: I92a27b946d5d60c2e15cdedb7faf349571f13c81
diff --git a/aos/network/message_bridge_protocol.h b/aos/network/message_bridge_protocol.h
index 485defe..23a6e0f 100644
--- a/aos/network/message_bridge_protocol.h
+++ b/aos/network/message_bridge_protocol.h
@@ -31,6 +31,10 @@
 // The stream on which timestamp replies are sent.
 constexpr size_t kTimestampStream() { return 1; }
 
+// Overhead constant for headers.  Both remote timestamps and the extra context
+// inside RemoteData need to fit inside this.
+constexpr size_t kHeaderSizeOverhead() { return 208u; }
+
 // Builds up a subscription request for my_node to remote_name.
 aos::FlatbufferDetachedBuffer<aos::message_bridge::Connect> MakeConnectMessage(
     const Configuration *config, const Node *my_node,