[Cocoa] Check message IDs when decoding Mach messages in IPC::Connection

[Cocoa] Check message IDs when decoding Mach messages in IPC::Connection
https://bugs.webkit.org/show_bug.cgi?id=192300
rdar://problem/46215523

Reviewed by Chris Dumez.

* Platform/IPC/mac/ConnectionMac.mm:
(IPC::Connection::sendOutgoingMessage): Use message IDs other than 0 and 1 for
the messages, as suggested by an expert at Apple.
(IPC::createMessageDecoder): Update for the new message IDs.
(IPC::Connection::receiveSourceEventHandler): Ignore messages with unexpected
message IDs.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@238800 268f45cc-cd09-0410-ab3c-d52691b4dbfc