Workers: Free memory when worker is terminated before thread creation

In the case a dedicated worker is terminated before the backing thread
has been created the message proxy would never be cleaned up. With
unified heap this means that any wrappers may keep on to old window
objects.

Bug: 928781, 843903
Change-Id: Ic0f1968609d259a2eea9a4c90d47f95729b4ce79
Reviewed-on: https://chromium-review.googlesource.com/c/1458217Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629949}

third_party/blink/renderer/core/workers/threaded_messaging_proxy_base.cc

@@ -146,8 +146,12 @@ void ThreadedMessagingProxyBase::TerminateGlobalScope() {
 
   terminate_sync_load_event_.Signal();
 
-  if (!worker_thread_)
+  if (!worker_thread_) {
+    // Worker has been terminated before any backing thread was attached to the
+    // messaging proxy.
+    keep_alive_.Clear();
     return;
+  }
   worker_thread_->Terminate();
   DevToolsAgent::WorkerThreadTerminated(execution_context_.Get(),
                                         worker_thread_.get());