Switch DRM platform to using a separate thread

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

Index: ui/ozone/platform/drm/gpu/gbm_buffer.h
diff --git a/ui/ozone/platform/drm/gpu/gbm_buffer.h b/ui/ozone/platform/drm/gpu/gbm_buffer.h
index ecc385e34b53e986c43ba2d1d8a6e8e994257eb7..cd36ca790590f82db43d15c4984017f572c79620 100644
--- a/ui/ozone/platform/drm/gpu/gbm_buffer.h
+++ b/ui/ozone/platform/drm/gpu/gbm_buffer.h
@@ -10,13 +10,14 @@
 #include "ui/gfx/buffer_types.h"
 #include "ui/gfx/geometry/size.h"
 #include "ui/ozone/platform/drm/gpu/gbm_buffer_base.h"
-#include "ui/ozone/platform/drm/gpu/screen_manager.h"
 #include "ui/ozone/public/native_pixmap.h"
 
 struct gbm_bo;
 
 namespace ui {
 
+class DrmThread;
+class DrmWindowProxy;
 class GbmDevice;
 
 class GbmBuffer : public GbmBufferBase {
@@ -41,8 +42,7 @@ class GbmBuffer : public GbmBufferBase {
 
 class GbmPixmap : public NativePixmap {
  public:
-  GbmPixmap(const scoped_refptr<GbmBuffer>& buffer,
-            ScreenManager* screen_manager);
+  GbmPixmap(const scoped_refptr<GbmBuffer>& buffer, DrmThread* drm_thread);
   bool Initialize();
   void SetScalingCallback(const ScalingCallback& scaling_callback) override;
   scoped_refptr<NativePixmap> GetScaledPixmap(gfx::Size new_size) override;
@@ -69,7 +69,11 @@ class GbmPixmap : public NativePixmap {
   scoped_refptr<GbmBuffer> buffer_;
   int dma_buf_ = -1;
 
-  ScreenManager* screen_manager_;  // Not owned.
+  DrmThread* drm_thread_;  // Not owned.
+
+  // Since buffers are likely to be shown on the same window, keep a cached
+  // window proxy so we don't have to get this every time the buffer is used.
+  scoped_ptr<DrmWindowProxy> cached_window_;
 
   ScalingCallback scaling_callback_;