VR: Destroy scene and scene manager on the thread they were created.

chrome/browser/android/vr_shell/vr_gl_thread.cc

 // Copyright 2016 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 "chrome/browser/android/vr_shell/vr_gl_thread.h"
 
 #include <utility>
 
 #include "chrome/browser/android/vr_shell/ui_scene.h"
 #include "chrome/browser/android/vr_shell/ui_scene_manager.h"
@@ skipping 26 matching lines @@
   vr_shell_gl_ = base::MakeUnique<VrShellGl>(
       std::move(weak_vr_shell_), std::move(main_thread_task_runner_), gvr_api_,
       initially_web_vr_, reprojected_rendering_, scene_.get());
   weak_vr_shell_gl_ = vr_shell_gl_->GetWeakPtr();
   weak_scene_manager_ = scene_manager_->GetWeakPtr();
   vr_shell_gl_->Initialize();
 }
 
 void VrGLThread::CleanUp() {
   vr_shell_gl_.reset();
+  scene_manager_.reset();
+  scene_.reset();
 }
 
 }  // namespace vr_shell