Fix black screen when switching primary display without bounds change

ui/ozone/platform/drm/gpu/mock_scanout_buffer.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/mock_scanout_buffer.h"
 
 namespace ui {
 
 MockScanoutBuffer::MockScanoutBuffer(const gfx::Size& size, uint32_t format)
     : size_(size), format_(format) {}
 
 MockScanoutBuffer::~MockScanoutBuffer() {}
 
 uint32_t MockScanoutBuffer::GetFramebufferId() const {
   return 1;
 }
 
 uint32_t MockScanoutBuffer::GetHandle() const {
   return 0;
 }
 
 gfx::Size MockScanoutBuffer::GetSize() const {
   return size_;
 }
 
 uint32_t MockScanoutBuffer::GetFramebufferPixelFormat() const {
   return format_;
 }
 
+const DrmDevice* MockScanoutBuffer::GetDrmDevice() const {
+  return nullptr;
+}
+
 bool MockScanoutBuffer::RequiresGlFinish() const {
   return false;
 }
 
 }  // namespace ui