Don't synthesize mousemove events if the mouse cursor is not on the widget

ui/views/widget/widget.h

Index: ui/views/widget/widget.h
diff --git a/ui/views/widget/widget.h b/ui/views/widget/widget.h
index 320332bec2461bd309d694e3651eabc7ba38c712..ca9a2e6ef74c6c98ca3a5497e14137e54f1cc658 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -927,6 +927,9 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
   // Whether native widget has been destroyed.
   bool native_widget_destroyed_;
 
+  // Whether the mouse cursor is on this widget.
+  bool is_mouse_on_widget_;
+
   // TODO(beng): Remove NativeWidgetGtk's dependence on these:
   // If true, the mouse is currently down.
   bool is_mouse_button_pressed_;