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 e6881de0befab8179345776d1ca16abefb1ad7af..47e56235bd7aa10265cfb8dd915286f5702a8103 100644
--- a/ui/ozone/BUILD.gn
+++ b/ui/ozone/BUILD.gn
@@ -43,6 +43,11 @@ if (ozone_platform_cast) {
   ozone_platform_deps += [ "platform/cast" ]
 }
 
+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"