Create new class "CastTransport", which encapsulates the message read and write event loops.

extensions/browser/api/cast_channel/cast_message_util.h

Index: extensions/browser/api/cast_channel/cast_message_util.h
diff --git a/extensions/browser/api/cast_channel/cast_message_util.h b/extensions/browser/api/cast_channel/cast_message_util.h
index 9080639fe736a755d4da03cd6979ceec947f0cea..eb1c86af01dd7584873ce954e5273ba79748fd85 100644
--- a/extensions/browser/api/cast_channel/cast_message_util.h
+++ b/extensions/browser/api/cast_channel/cast_message_util.h
@@ -19,6 +19,9 @@ struct MessageInfo;
 bool MessageInfoToCastMessage(const MessageInfo& message,
                               CastMessage* message_proto);
 
+// Checks if the contents of |message_proto| are semantically valid.
+bool IsCastMessageValid(const CastMessage& message_proto);
+
 // Fills |message| from |message_proto| and returns true on success.
 bool CastMessageToMessageInfo(const CastMessage& message_proto,
                               MessageInfo* message);