Add PlatformWindow/Event related code for Ozone X11.

ui/ozone/BUILD.gn

Index: ui/ozone/BUILD.gn
diff --git a/ui/ozone/BUILD.gn b/ui/ozone/BUILD.gn
index 9ef3103b8f68c9d8d59e46939a0f1466999d1bfd..fb647e0ad99a8043aa4524b40dbd2e31f6ad190a 100644
--- a/ui/ozone/BUILD.gn
+++ b/ui/ozone/BUILD.gn
@@ -48,6 +48,11 @@ if (ozone_platform_wayland) {
   ozone_platform_deps += [ "platform/wayland" ]
 }
 
+if (ozone_platform_x11) {
+  ozone_platforms += [ "x11" ]
+  ozone_platform_deps += [ "platform/x11" ]
+}
+
 platform_list_cc_file = "$target_gen_dir/platform_list.cc"
 platform_list_h_file = "$target_gen_dir/platform_list.h"
 platform_list_txt_file = "$target_gen_dir/platform_list.txt"