Destroy tooltip manager if the native window is destroyed.

views/widget/native_widget_aura.cc

Index: views/widget/native_widget_aura.cc
diff --git a/views/widget/native_widget_aura.cc b/views/widget/native_widget_aura.cc
index 11fcb076f8cf1dc2c295e175dc165dfca9341707..af8ab47fe33c9da26501d95da310651f422cfbc3 100644
--- a/views/widget/native_widget_aura.cc
+++ b/views/widget/native_widget_aura.cc
@@ -535,6 +535,9 @@ void NativeWidgetAura::OnPaint(gfx::Canvas* canvas) {
 
 void NativeWidgetAura::OnWindowDestroying() {
   delegate_->OnNativeWidgetDestroying();
+
+  // If the aura::Window is destroyed, we can no longer show tooltips.
+  tooltip_manager_.reset();
 }
 
 void NativeWidgetAura::OnWindowDestroyed() {