webkitpy: prepare to delete message_broker.py

webkitpy: prepare to delete message_broker.py
https://bugs.webkit.org/show_bug.cgi?id=78997

Reviewed by Eric Seidel.

This is part 1 of 3 changes to fix bug 78187. This change
prepares the code for merging message_broker.py into
manager_worker_broker.py; the tests in message_broker_unittest
are merged into manager_worker_broker_unittest, the symbols
needed by the tests are exported from manager_worker_broker.py,
and manager_worker_broker itself is updated to refer to the
symbols directly (without the module prefix).

* Scripts/webkitpy/layout_tests/controllers/manager_worker_broker.py:
(get):
(AbstractWorker):
(AbstractWorker.__init__):
(_ManagerConnection):
(_ManagerConnection.__init__):
(_WorkerConnection):
(_WorkerConnection.__init__):
* Scripts/webkitpy/layout_tests/controllers/manager_worker_broker_unittest.py:
(InterfaceTest.test_brokerclient_is_abstract):
(InterfaceTest.test_brokerclient_is_abstract.methods):
(MessageTest):
(MessageTest.test__no_body):
(MessageTest.test__body):
* Scripts/webkitpy/layout_tests/controllers/message_broker_unittest.py: Removed.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108376 268f45cc-cd09-0410-ab3c-d52691b4dbfc