[mojo] Use base::Pickle for Message storage.

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

Index: mojo/public/cpp/bindings/lib/message_internal.h
diff --git a/mojo/public/cpp/bindings/lib/message_internal.h b/mojo/public/cpp/bindings/lib/message_internal.h
index 97b96193ff6a2b6e5e7a7863652d926e5cf52432..8bc2e0da8a4d6b6113673e11724329a0c61c5ac1 100644
--- a/mojo/public/cpp/bindings/lib/message_internal.h
+++ b/mojo/public/cpp/bindings/lib/message_internal.h
@@ -35,13 +35,6 @@ struct MessageHeaderWithRequestID : MessageHeader {
 static_assert(sizeof(MessageHeaderWithRequestID) == 32,
               "Bad sizeof(MessageHeaderWithRequestID)");
 
-struct MessageData {
-  MessageHeader header;
-};
-
-static_assert(sizeof(MessageData) == sizeof(MessageHeader),
-              "Bad sizeof(MessageData)");
-
 #pragma pack(pop)
 
 }  // namespace internal