Makes a couple of tests reset state appropriately

content/browser/renderer_host/render_widget_host_unittest.cc

 // Copyright (c) 2012 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 "base/basictypes.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/shared_memory.h"
 #include "base/timer/timer.h"
@@ skipping 440 matching lines @@
 
   void TearDown() override {
     view_.reset();
     host_.reset();
     delegate_.reset();
     process_ = NULL;
     browser_context_.reset();
 
 #if defined(USE_AURA)
     aura::Env::DeleteInstance();
+    gfx::Screen::SetScreenInstance(gfx::SCREEN_TYPE_NATIVE, nullptr);
     screen_.reset();
 #endif
 #if defined(USE_AURA) || (defined(OS_MACOSX) && !defined(OS_IOS))
     if (IsDelegatedRendererEnabled())
       ImageTransportFactory::Terminate();
 #endif
 
     // Process all pending tasks to avoid leaks.
     base::MessageLoop::current()->RunUntilIdle();
   }
@@ skipping 997 matching lines @@
   // Having an initial size set means that the size information had been sent
   // with the reqiest to new up the RenderView and so subsequent WasResized
   // calls should not result in new IPC (unless the size has actually changed).
   host_->WasResized();
   EXPECT_FALSE(process_->sink().GetUniqueMessageMatching(ViewMsg_Resize::ID));
   EXPECT_EQ(initial_size_, host_->old_resize_params_->new_size);
   EXPECT_TRUE(host_->resize_ack_pending_);
 }
 
 }  // namespace content