Switch DRM platform to using a separate thread

ui/ozone/platform/drm/BUILD.gn

Index: ui/ozone/platform/drm/BUILD.gn
diff --git a/ui/ozone/platform/drm/BUILD.gn b/ui/ozone/platform/drm/BUILD.gn
index c345691575cdd053e6f902a895ead9020b4f52c0..f4eae526c398aa2b38c4a13014c8945584d390cb 100644
--- a/ui/ozone/platform/drm/BUILD.gn
+++ b/ui/ozone/platform/drm/BUILD.gn
@@ -45,10 +45,18 @@ source_set("gbm") {
     "gpu/drm_gpu_display_manager.h",
     "gpu/drm_gpu_platform_support.cc",
     "gpu/drm_gpu_platform_support.h",
+    "gpu/drm_thread.cc",
+    "gpu/drm_thread.h",
+    "gpu/drm_thread_message_proxy.cc",
+    "gpu/drm_thread_message_proxy.h",
+    "gpu/drm_thread_proxy.cc",
+    "gpu/drm_thread_proxy.h",
     "gpu/drm_vsync_provider.cc",
     "gpu/drm_vsync_provider.h",
     "gpu/drm_window.cc",
     "gpu/drm_window.h",
+    "gpu/drm_window_proxy.cc",
+    "gpu/drm_window_proxy.h",
     "gpu/gbm_buffer.cc",
     "gpu/gbm_buffer.h",
     "gpu/gbm_buffer_base.cc",
@@ -71,6 +79,8 @@ source_set("gbm") {
     "gpu/overlay_plane.h",
     "gpu/page_flip_request.cc",
     "gpu/page_flip_request.h",
+    "gpu/proxy_helpers.cc",
+    "gpu/proxy_helpers.h",
     "gpu/scanout_buffer.h",
     "gpu/screen_manager.cc",
     "gpu/screen_manager.h",