Reland: Fix crash when the source browser window is deleted during a drag

ui/views/widget/widget.cc

Index: ui/views/widget/widget.cc
diff --git a/ui/views/widget/widget.cc b/ui/views/widget/widget.cc
index 7f649c04025bffe95c5f1ab444279baf94f77572..a24f781df4d3ba81640a64d935d8547ca5bc183d 100644
--- a/ui/views/widget/widget.cc
+++ b/ui/views/widget/widget.cc
@@ -788,7 +788,14 @@ void Widget::RunShellDrag(View* view,
                           ui::DragDropTypes::DragEventSource source) {
   dragged_view_ = view;
   OnDragWillStart();
+
+  WidgetDeletionObserver widget_deletion_observer(this);
   native_widget_->RunShellDrag(view, data, location, operation, source);
+
+  // The widget may be destroyed during the drag operation.
+  if (!widget_deletion_observer.IsWidgetAlive())
+    return;
+
   // If the view is removed during the drag operation, dragged_view_ is set to
   // NULL.
   if (view && dragged_view_ == view) {