Get rid of MessageID::get()
https://bugs.webkit.org/show_bug.cgi?id=108235

Reviewed by Beth Dakin.

Just check for the message receiver name and message name explicitly instead.

* Platform/CoreIPC/MessageID.h:
(CoreIPC::MessageID::is):
* Shared/CoreIPCSupport/WebContextMessageKinds.h:
(WebContextLegacyMessage::messageReceiverName):
(WebContextLegacyMessage):
(WebContextLegacyMessage::postMessageMessageName):
(WebContextLegacyMessage::postSynchronousMessageMessageName):
* UIProcess/WebContext.cpp:
(WebKit::WebContext::didReceiveMessage):
(WebKit::WebContext::didReceiveSyncMessage):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141170 268f45cc-cd09-0410-ab3c-d52691b4dbfc