Switch DRM platform to using a separate thread

ui/ozone/platform/drm/gpu/drm_window.h

Index: ui/ozone/platform/drm/gpu/drm_window.h
diff --git a/ui/ozone/platform/drm/gpu/drm_window.h b/ui/ozone/platform/drm/gpu/drm_window.h
index ad92eccfaa6f6d10f444a1685e10dd9bb4c126cd..7764f4f0c954bc78cb8ba0de77f15a81099d8d2d 100644
--- a/ui/ozone/platform/drm/gpu/drm_window.h
+++ b/ui/ozone/platform/drm/gpu/drm_window.h
@@ -28,6 +28,7 @@ class Rect;
 namespace ui {
 
 class DrmBuffer;
+class DrmDevice;
 class DrmDeviceManager;
 class HardwareDisplayController;
 struct OverlayCheck_Params;
@@ -43,7 +44,7 @@ class ScreenManager;
 //
 // If there's no display whose bounds match the window's, the window is
 // disconnected and its contents will not be visible to the user.
-class OZONE_EXPORT DrmWindow {
+class OZONE_EXPORT DrmWindow : public base::SupportsWeakPtr<DrmWindow> {
  public:
   DrmWindow(gfx::AcceleratedWidget widget,
             DrmDeviceManager* device_manager,
@@ -89,7 +90,7 @@ class OZONE_EXPORT DrmWindow {
   // is once again ready.
   void QueueOverlayPlane(const OverlayPlane& plane);
 
-  bool SchedulePageFlip(const SwapCompletionCallback& callback);
+  void SchedulePageFlip(const SwapCompletionCallback& callback);
   bool TestPageFlip(const std::vector<OverlayCheck_Params>& planes,
                     ScanoutBufferGenerator* buffer_generator);