Fix AshWindowTreeHost shutdown for Ozone X11.

ui/platform_window/x11/x11_window_ozone.cc

Index: ui/platform_window/x11/x11_window_ozone.cc
diff --git a/ui/platform_window/x11/x11_window_ozone.cc b/ui/platform_window/x11/x11_window_ozone.cc
index c3051cb5eafc49d4acc363a43f748488ede002db..c7a016e46f25755a679e1600b64f441f8b6fb4bd 100644
--- a/ui/platform_window/x11/x11_window_ozone.cc
+++ b/ui/platform_window/x11/x11_window_ozone.cc
@@ -16,22 +16,28 @@
 
 namespace ui {
 
-X11WindowOzone::X11WindowOzone(X11EventSourceLibevent* event_source,
-                               X11WindowManagerOzone* window_manager,
+X11WindowOzone::X11WindowOzone(X11WindowManagerOzone* window_manager,
                                PlatformWindowDelegate* delegate,
                                const gfx::Rect& bounds)
-    : X11WindowBase(delegate, bounds),
-      event_source_(event_source),
-      window_manager_(window_manager) {
-  DCHECK(event_source_);
+    : X11WindowBase(delegate, bounds), window_manager_(window_manager) {
   DCHECK(window_manager);
-  event_source_->AddPlatformEventDispatcher(this);
-  event_source_->AddXEventDispatcher(this);
+  auto* event_source = X11EventSourceLibevent::GetInstance();
+  if (event_source) {
+    event_source->AddPlatformEventDispatcher(this);
+    event_source->AddXEventDispatcher(this);
+  }
 }
 
 X11WindowOzone::~X11WindowOzone() {
-  event_source_->RemovePlatformEventDispatcher(this);
-  event_source_->RemoveXEventDispatcher(this);
+  X11WindowOzone::PrepareForShutdown();
+}
+
+void X11WindowOzone::PrepareForShutdown() {
+  auto* event_source = X11EventSourceLibevent::GetInstance();
+  if (event_source) {
+    event_source->RemovePlatformEventDispatcher(this);
+    event_source->RemoveXEventDispatcher(this);
+  }
 }
 
 void X11WindowOzone::SetCapture() {