Make Mandoline shut down cleanly.

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 14e6fdf0b282421af5e88e7bf8c6c27032543337..5e8f41118b95f3b1b083c0f31d5b1948957f8ff5 100644
--- a/ui/platform_window/x11/x11_window.cc
+++ b/ui/platform_window/x11/x11_window.cc
@@ -46,7 +46,8 @@ X11Window::X11Window(PlatformWindowDelegate* delegate)
       xwindow_(None),
       xroot_window_(DefaultRootWindow(xdisplay_)),
       atom_cache_(xdisplay_, kAtomsToCache),
-      window_mapped_(false) {
+      window_mapped_(false),
+      destroyed_(false) {
   CHECK(delegate_);
   TouchFactory::SetTouchDeviceListFromCommandLine();
 }
@@ -56,14 +57,20 @@ X11Window::~X11Window() {
 }
 
 void X11Window::Destroy() {
-  delegate_->OnClosed();
-  if (xwindow_ == None)
+  if (destroyed_)
     return;
 
-  // Stop processing events.
-  PlatformEventSource::GetInstance()->RemovePlatformEventDispatcher(this);
-  XDestroyWindow(xdisplay_, xwindow_);
-  xwindow_ = None;
+  destroyed_ = true;
+
+  if (xwindow_ != None) {
+    // Stop processing events.
+    PlatformEventSource::GetInstance()->RemovePlatformEventDispatcher(this);
+    XDestroyWindow(xdisplay_, xwindow_);
+    xwindow_ = None;
+  }
+
+  delegate_->OnClosed();
+  // |this| could be deleted now.
 }
 
 void X11Window::ProcessXInput2Event(XEvent* xev) {