Make SecItemShimProxy be a WorkQueueMessageReceiver
https://bugs.webkit.org/show_bug.cgi?id=109719

Reviewed by Sam Weinig.

This adds a WantsConnection message attribute to be used for messages whose handlers
should take the connection the message was delivered to.

* Platform/CoreIPC/HandleMessage.h:
(CoreIPC::handleMessage):
Add new handleMessage overload.

* Scripts/webkit2/messages.py:
(async_message_statement):
(generate_message_handler):
Handle the WantsMessage attribute.

* UIProcess/mac/SecItemShimProxy.cpp:
(WebKit::SecItemShimProxy::shared):
Use dispatch_once and adoptRef.

(WebKit::SecItemShimProxy::SecItemShimProxy):
Initialize the queue.

(WebKit::SecItemShimProxy::initializeConnection):
Add the proxy as a work queue message receiver.

(WebKit::SecItemShimProxy::secItemRequest):
This no longer needs to call out to a dispatch queue, it's already on a queue.

* UIProcess/mac/SecItemShimProxy.messages.in:
This doesn't need to be a legacy receiver. Also, add the WantsConnection message.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142763 268f45cc-cd09-0410-ab3c-d52691b4dbfc