[V8] Web Inspector: simplify worker debugger implementation

[V8] Web Inspector: simplify worker debugger implementation
https://bugs.webkit.org/show_bug.cgi?id=79085

There is one instance of WorkerScriptDebugServer per worker thread. We don't
need to resolve WorkerContext from V8::Context anymore.

Reviewed by Pavel Feldman.

* bindings/v8/WorkerScriptDebugServer.cpp:
(WebCore::WorkerScriptDebugServer::WorkerScriptDebugServer):
(WebCore::WorkerScriptDebugServer::addListener):
(WebCore::WorkerScriptDebugServer::removeListener):
(WebCore::WorkerScriptDebugServer::getDebugListenerForContext):
(WebCore::WorkerScriptDebugServer::runMessageLoopOnPause):
* bindings/v8/WorkerScriptDebugServer.h:
(WorkerScriptDebugServer):
* inspector/WorkerDebuggerAgent.cpp:
(WebCore::WorkerDebuggerAgent::WorkerDebuggerAgent):
(WebCore::WorkerDebuggerAgent::startListeningScriptDebugServer):
(WebCore::WorkerDebuggerAgent::stopListeningScriptDebugServer):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108324 268f45cc-cd09-0410-ab3c-d52691b4dbfc