Add PlatformWindow/Event related code for Ozone X11.

ui/platform_window/x11/BUILD.gn

Index: ui/platform_window/x11/BUILD.gn
diff --git a/ui/platform_window/x11/BUILD.gn b/ui/platform_window/x11/BUILD.gn
index 4775aa771449011f02f181f1dc2c3a478a3b2824..8bc79a856d25f8092e10005ab29a59443fc5574d 100644
--- a/ui/platform_window/x11/BUILD.gn
+++ b/ui/platform_window/x11/BUILD.gn
@@ -3,8 +3,9 @@
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
+import("//ui/ozone/ozone.gni")
 
-assert(use_x11)
+assert(use_x11 || ozone_platform_x11)
 
 component("x11") {
   output_name = "x11_window"
@@ -29,4 +30,10 @@ component("x11") {
     "x11_window.h",
     "x11_window_export.h",
   ]
+
+  if (use_x11) {
+    sources += [ "x11_window_linux.cc" ]
+  } else if (ozone_platform_x11) {
+    sources += [ "x11_window_ozone.cc" ]
+  }
 }