nrwt: make the worker class stand alone with a cleaner interface

nrwt: make the worker class stand alone with a cleaner interface
https://bugs.webkit.org/show_bug.cgi?id=90409

Reviewed by Ojan Vafai.

Currently the Worker class derives from AbstractWorker, which is
kind of crufty and awkward; it would be better if we did not
rely on shared state.

This change changes that so that Worker derives from object, and
exposes the following interface:
  __init__() - called in the manager process
  safe_init() - called in the worker process to initialize
    unpicklable state
  handle() - a single routine to handle all messages
  cleanup() - called so the worker can clean up

Also, all of the "administrative" messages that are handled by
the worker (notification of start/stop/etc.) move into
manager_worker_broker - this reduces worker.py to just handling
the mechanics of actually running each test.

For the moment, we do this by creating Yet Another wrapper/proxy
class in manager_worker_broker, but this will get simpler
shortly when the rest of m_w_b is cleaned up.

With this change worker is now in its new form but there will be
a follow-on change that cleans up some names and other minor
things.

This change is again mostly just moving things around and should
be covered by the (updated) existing tests.

* Scripts/webkitpy/layout_tests/controllers/manager_worker_broker.py:
(get):
(AbstractWorker.__init__):
(AbstractWorker.run):
(AbstractWorker):
(AbstractWorker.handle_stop):
(AbstractWorker.handle_test_list):
(AbstractWorker.yield_to_broker):
(AbstractWorker.post_message):
(_WorkerConnection.__init__):
(_Process.run):
* Scripts/webkitpy/layout_tests/controllers/manager_worker_broker_unittest.py:
(_TestWorker):
(_TestWorker.__init__):
(_TestWorker.name):
(_TestWorker.cleanup):
(_TestWorker.handle):
(_TestWorker.safe_init):
(_TestWorker.stop):
(_TestsMixin.handle_finished_test):
(_TestsMixin.setUp):
(_TestsMixin.test_cancel):
(_TestsMixin.test_done):
* Scripts/webkitpy/layout_tests/controllers/worker.py:
(Worker):
(Worker.__init__):
(Worker.safe_init):
(Worker.handle):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121809 268f45cc-cd09-0410-ab3c-d52691b4dbfc