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

ui/views/widget/widget.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/widget/widget.h"
 
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 770 matching lines @@
   return native_widget_private()->GetHostInputMethod();
 }
 
 void Widget::RunShellDrag(View* view,
                           const ui::OSExchangeData& data,
                           const gfx::Point& location,
                           int operation,
                           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) {
     dragged_view_ = NULL;
     view->OnDragDone();
   }
   OnDragComplete();
 }
 
 void Widget::SchedulePaintInRect(const gfx::Rect& rect) {
@@ skipping 715 matching lines @@
 
 ////////////////////////////////////////////////////////////////////////////////
 // internal::NativeWidgetPrivate, NativeWidget implementation:
 
 internal::NativeWidgetPrivate* NativeWidgetPrivate::AsNativeWidgetPrivate() {
   return this;
 }
 
 }  // namespace internal
 }  // namespace views