Two changes to desktop aura for tooltips:

ui/views/corewm/tooltip_controller.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/corewm/tooltip_controller.h"
 
 #include <vector>
 
 #include "base/command_line.h"
 #include "base/location.h"
@@ skipping 238 matching lines @@
   // simply simulate a timeout.
   if (tooltip_shown_timer_.IsRunning()) {
     tooltip_shown_timer_.Stop();
     TooltipShownTimerFired();
   }
 }
 
 void TooltipController::OnMouseEvent(ui::MouseEvent* event) {
   aura::Window* target = static_cast<aura::Window*>(event->target());
   switch (event->type()) {
+    case ui::ET_MOUSE_EXITED:
+      target = NULL;
+      // Fall through.
     case ui::ET_MOUSE_MOVED:
     case ui::ET_MOUSE_DRAGGED:
       if (tooltip_window_ != target) {
         if (tooltip_window_)
           tooltip_window_->RemoveObserver(this);
         tooltip_window_ = target;
-        tooltip_window_->AddObserver(this);
+        if (tooltip_window_)
+          tooltip_window_->AddObserver(this);
       }
       curr_mouse_loc_ = event->location();
       if (tooltip_timer_.IsRunning())
         tooltip_timer_.Reset();
 
       if (GetTooltip()->IsVisible())
         UpdateIfRequired();
       break;
     case ui::ET_MOUSE_PRESSED:
       if ((event->flags() & ui::EF_IS_NON_CLIENT) == 0) {
@@ skipping 252 matching lines @@
 int TooltipController::GetTooltipShownTimeout() {
   std::map<aura::Window*, int>::const_iterator it =
       tooltip_shown_timeout_map_.find(tooltip_window_);
   if (it == tooltip_shown_timeout_map_.end())
     return kDefaultTooltipShownTimeoutMs;
   return it->second;
 }
 
 }  // namespace corewm
 }  // namespace views