Switch DRM platform to using a separate thread

ui/ozone/platform/drm/gpu/screen_manager.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 "ui/ozone/platform/drm/gpu/screen_manager.h"
 
 #include <xf86drmMode.h>
 
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "ui/gfx/geometry/point.h"
@@ skipping 217 matching lines @@
   DCHECK(window) << "Attempting to remove non-existing window for " << widget;
   UpdateControllerToWindowMapping();
   return window.Pass();
 }
 
 DrmWindow* ScreenManager::GetWindow(gfx::AcceleratedWidget widget) {
   WidgetToWindowMap::iterator it = window_map_.find(widget);
   if (it != window_map_.end())
     return it->second;
 
-  NOTREACHED() << "Attempting to get non-existing window for " << widget;
   return nullptr;
 }
 
 ScreenManager::HardwareDisplayControllers::iterator
 ScreenManager::FindDisplayController(const scoped_refptr<DrmDevice>& drm,
                                      uint32_t crtc) {
   for (HardwareDisplayControllers::iterator it = controllers_.begin();
        it != controllers_.end(); ++it) {
     if ((*it)->HasCrtc(drm, crtc))
       return it;
@@ skipping 136 matching lines @@
 DrmWindow* ScreenManager::FindWindowAt(const gfx::Rect& bounds) const {
   for (auto pair : window_map_) {
     if (pair.second->bounds() == bounds)
       return pair.second;
   }
 
   return nullptr;
 }
 
 }  // namespace ui