Switch DRM platform to using a separate thread

ui/ozone/platform/drm/gpu/proxy_helpers.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ui/ozone/platform/drm/gpu/proxy_helpers.h"
+
+#include "base/synchronization/waitable_event.h"
+
+namespace ui {
+
+namespace {
+
+void OnRunPostedTaskAndSignal(const base::Closure& callback,
+                              base::WaitableEvent* wait) {
+  callback.Run();
+  wait->Signal();
+}
+
+}  // namespace
+
+void PostSyncTask(
+    const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
+    const base::Closure& callback) {
+  base::WaitableEvent wait(false, false);
+  bool success = task_runner->PostTask(
+      FROM_HERE, base::Bind(OnRunPostedTaskAndSignal, callback, &wait));
+  if (success)
+    wait.Wait();
+}
+
+}  // namespace ui