Makes a couple of tests reset state appropriately

chrome/test/base/view_event_test_platform_part_ash.cc

 // Copyright 2014 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/test/base/view_event_test_platform_part.h"
 
+#include "base/memory/scoped_ptr.h"
 #include "ui/aura/env.h"
 #include "ui/gfx/screen.h"
 #include "ui/views/widget/desktop_aura/desktop_screen.h"
 #include "ui/wm/core/wm_state.h"
 
 namespace {
 
 // ChromeViewsTestHelper implementation for non-ChromeOS environments, where the
 // Ash desktop environment is available (use_ash=1, chromeos=0).
 class ViewEventTestPlatformPartAsh : public ViewEventTestPlatformPart {
  public:
   explicit ViewEventTestPlatformPartAsh(ui::ContextFactory* context_factory);
   ~ViewEventTestPlatformPartAsh() override;
 
   // Overridden from ViewEventTestPlatformPart:
   gfx::NativeWindow GetContext() override {
     return NULL;  // No context, so that desktop tree hosts are used by default.
   }
 
  private:
+  scoped_ptr<gfx::Screen> screen_;
   wm::WMState wm_state_;
 
   DISALLOW_COPY_AND_ASSIGN(ViewEventTestPlatformPartAsh);
 };
 
 ViewEventTestPlatformPartAsh::ViewEventTestPlatformPartAsh(
-    ui::ContextFactory* context_factory) {
+    ui::ContextFactory* context_factory)
+    : screen_(views::CreateDesktopScreen()) {
   // http://crbug.com/154081 use ash::Shell code path below on win_ash bots when
   // interactive_ui_tests is brought up on that platform.
-  gfx::Screen::SetScreenInstance(gfx::SCREEN_TYPE_NATIVE,
-                                 views::CreateDesktopScreen());
+  gfx::Screen::SetScreenInstance(gfx::SCREEN_TYPE_NATIVE, screen_.get());
   aura::Env::CreateInstance(true);
   aura::Env::GetInstance()->set_context_factory(context_factory);
 }
 
 ViewEventTestPlatformPartAsh::~ViewEventTestPlatformPartAsh() {
   aura::Env::DeleteInstance();
+  gfx::Screen::SetScreenInstance(gfx::SCREEN_TYPE_NATIVE, nullptr);
 }
 
 }  // namespace
 
 // static
 ViewEventTestPlatformPart* ViewEventTestPlatformPart::Create(
     ui::ContextFactory* context_factory) {
   return new ViewEventTestPlatformPartAsh(context_factory);
 }