d8 workers: fix race on quit() with context_mutex_

When quit() is called, d8 shell exits without cleanup. If a worker is running,
it might be holding the context_mutex_, which if destroyed will DCHECK.

BUG=4279
R=jarin@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1231473002

Cr-Commit-Position: refs/heads/master@{#29557}

src/d8.cc

@@ -810,6 +810,9 @@ void Shell::WorkerTerminate(const v8::FunctionCallbackInfo<v8::Value>& args) {
 
 void Shell::Quit(const v8::FunctionCallbackInfo<v8::Value>& args) {
   int exit_code = args[0]->Int32Value();
+#ifndef V8_SHARED
+  CleanupWorkers();
+#endif  // !V8_SHARED
   OnExit(args.GetIsolate());
   exit(exit_code);
 }

test/mjsunit/regress/regress-4279.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+if (this.Worker && this.quit) {
+  try {
+      new Function(new Worker("55"));
+  } catch(err) {}
+
+  quit();
+}