Prevent a crash if the window was deleted while in a drag runloop

ash/wm/toplevel_window_event_handler.cc

Index: ash/wm/toplevel_window_event_handler.cc
diff --git a/ash/wm/toplevel_window_event_handler.cc b/ash/wm/toplevel_window_event_handler.cc
index 067e23875a9b8f5da41b43e5c55c461bbf28c4ca..cfc8ebffc70ae4d1cfa6270e9281e755188411f0 100644
--- a/ash/wm/toplevel_window_event_handler.cc
+++ b/ash/wm/toplevel_window_event_handler.cc
@@ -14,6 +14,7 @@
 #include "ui/aura/env.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_event_dispatcher.h"
+#include "ui/aura/window_tracker.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/base/cursor/cursor.h"
 #include "ui/base/hit_test.h"
@@ -94,13 +95,16 @@ aura::client::WindowMoveResult ToplevelWindowEventHandler::RunMoveLoop(
   wm::WindowState* window_state = wm::GetWindowState(source);
   const bool window_position_managed = window_state->window_position_managed();
   window_state->set_window_position_managed(false);
+  aura::WindowTracker tracker({source});
 
   run_loop.Run();
 
   if (!weak_ptr)
     return aura::client::MOVE_CANCELED;
 
-  window_state->set_window_position_managed(window_position_managed);
+  // Make sure the window hasn't been deleted.
+  if (tracker.Contains(source))
+    window_state->set_window_position_managed(window_position_managed);
 
   in_move_loop_ = false;
   return result == wm::WmToplevelWindowEventHandler::DragResult::SUCCESS