Make it possible for waitForAndDispatchImmediately to bail if a sync message comes...

Make it possible for waitForAndDispatchImmediately to bail if a sync message comes in from the other direction
https://bugs.webkit.org/show_bug.cgi?id=133708

Reviewed by Anders Carlsson.

* Platform/IPC/Connection.cpp:
(IPC::WaitForMessageState):
(IPC::Connection::Connection):
(IPC::Connection::waitForMessage):
(IPC::Connection::processIncomingMessage):
(IPC::Connection::connectionDidClose):
* Platform/IPC/Connection.h:
(IPC::Connection::waitForAndDispatchImmediately):
Remove the waitForMessageMap, and assert that we're only ever waiting for one message at a time.
This simplifies this code a bit, and we never wait on more than one message at a time, so it was unnecessary.

Add a flag to waitForAndDispatchImmediately, InterruptWaitingIfSyncMessageArrives, which will cause
waitForAndDispatchImmediately to bail if a sync message arrives, to avoid pointlessly blocking both processes
for the entire timeout.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@169825 268f45cc-cd09-0410-ab3c-d52691b4dbfc