C++ bindings: separate out serialization source set, have "mojom" targets optionally use serializat…

mojo/public/cpp/bindings/interface_request.h

Index: mojo/public/cpp/bindings/interface_request.h
diff --git a/mojo/public/cpp/bindings/interface_request.h b/mojo/public/cpp/bindings/interface_request.h
index b2b2ad3942b9aee4ff16e6987193be17a7738ff0..005f9b676c93d7a66ab92d8cce78d69329ada98d 100644
--- a/mojo/public/cpp/bindings/interface_request.h
+++ b/mojo/public/cpp/bindings/interface_request.h
@@ -5,10 +5,16 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_INTERFACE_REQUEST_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_INTERFACE_REQUEST_H_
 
-#include "mojo/public/cpp/bindings/interface_ptr.h"
+#include "mojo/public/cpp/system/message_pipe.h"
 
 namespace mojo {
 
+template <typename I>
+class InterfacePtr;
+
+template <typename I>
+class InterfacePtrInfo;
+
 // Represents a request from a remote client for an implementation of Interface
 // over a specified message pipe. The implementor of the interface should
 // remove the message pipe by calling PassMessagePipe() and bind it to the