Remove almost everything from MessageID
https://bugs.webkit.org/show_bug.cgi?id=108244

Reviewed by Beth Dakin.

* Platform/CoreIPC/MessageID.h:
(CoreIPC::MessageID::MessageID):
* Platform/CoreIPC/mac/ConnectionMac.cpp:
(CoreIPC):
(CoreIPC::Connection::sendOutgoingMessage):
(CoreIPC::createMessageDecoder):
(CoreIPC::Connection::receiveSourceEventHandler):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141177 268f45cc-cd09-0410-ab3c-d52691b4dbfc