Revert 124461 - Remove the singleton instance get/delete methods from RootWindow (yay)

ui/aura/root_window_host_linux.cc

Index: ui/aura/root_window_host_linux.cc
===================================================================
--- ui/aura/root_window_host_linux.cc	(revision 124478)
+++ ui/aura/root_window_host_linux.cc	(working copy)
@@ -305,6 +305,8 @@
   if (base::MessagePumpForUI::HasXInput2())
     ui::TouchFactory::GetInstance()->SetupXI2ForXWindow(xwindow_);
 
+  MessageLoopForUI::current()->AddDestructionObserver(this);
+
   // Initialize invisible cursor.
   char nodata[] = { 0, 0, 0, 0, 0, 0, 0, 0 };
   XColor black;
@@ -326,6 +328,8 @@
   ui::GetXCursor(ui::kCursorClearXCursorCache);
 
   XFreeCursor(xdisplay_, invisible_cursor_);
+
+  MessageLoopForUI::current()->RemoveDestructionObserver(this);
 }
 
 base::MessagePumpDispatcher::DispatchStatus RootWindowHostLinux::Dispatch(
@@ -620,6 +624,10 @@
   XSendEvent(xdisplay_, xwindow_, False, 0, &xevent);
 }
 
+void RootWindowHostLinux::WillDestroyCurrentMessageLoop() {
+  aura::RootWindow::DeleteInstance();
+}
+
 bool RootWindowHostLinux::IsWindowManagerPresent() {
   // Per ICCCM 2.8, "Manager Selections", window managers should take ownership
   // of WM_Sn selections (where n is a screen number).