Fix AshWindowTreeHost shutdown for Ozone X11.

ui/platform_window/x11/x11_window.cc

Index: ui/platform_window/x11/x11_window.cc
diff --git a/ui/platform_window/x11/x11_window.cc b/ui/platform_window/x11/x11_window.cc
index 3c4ee2a6e4b2dd6c489d1d64591f33d25a8c0ca5..64bd7a8f1ae3a566a5e1d5c89299915026ff8d7b 100644
--- a/ui/platform_window/x11/x11_window.cc
+++ b/ui/platform_window/x11/x11_window.cc
@@ -24,6 +24,10 @@ X11Window::X11Window(PlatformWindowDelegate* delegate, const gfx::Rect& bounds)
 }
 
 X11Window::~X11Window() {
+  X11Window::PrepareForShutdown();
+}
+
+void X11Window::PrepareForShutdown() {
   PlatformEventSource::GetInstance()->RemovePlatformEventDispatcher(this);
 }