Add NativePixmapHandle type & interface for exporting them

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 51a883904a03ce995bb5c3700b073fda3f295be2..576e45a5e69690590b11170482928aa2873b5030 100644
--- a/ui/ozone/platform/drm/gpu/gbm_buffer.h
+++ b/ui/ozone/platform/drm/gpu/gbm_buffer.h
@@ -51,6 +51,7 @@ class GbmPixmap : public NativePixmap {
                             gfx::OverlayTransform plane_transform,
                             const gfx::Rect& display_bounds,
                             const gfx::RectF& crop_rect) override;
+  gfx::NativePixmapHandle ExportHandle() override;
 
   scoped_refptr<GbmBuffer> buffer() { return buffer_; }