WebUI: prevent WebContent to hold an invalid pointer.

chrome/browser/ui/webui/print_preview/print_preview_ui.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 "chrome/browser/ui/webui/print_preview/print_preview_ui.h"
 
 #include <map>
 #include <utility>
 #include <vector>
 
@@ skipping 636 matching lines @@
 void PrintPreviewUI::OnHidePreviewDialog() {
   WebContents* preview_dialog = web_ui()->GetWebContents();
   printing::BackgroundPrintingManager* background_printing_manager =
       g_browser_process->background_printing_manager();
   if (background_printing_manager->HasPrintPreviewDialog(preview_dialog))
     return;
 
   ConstrainedWebDialogDelegate* delegate = GetConstrainedDelegate();
   if (!delegate)
     return;
-  delegate->ReleaseWebContentsOnDialogClose();
-  background_printing_manager->OwnPrintPreviewDialog(preview_dialog);
+  std::unique_ptr<content::WebContents> preview_contents =
+      delegate->ReleaseWebContents();
+  DCHECK_EQ(preview_dialog, preview_contents.get());
+  background_printing_manager->OwnPrintPreviewDialog(
+      preview_contents.release());
   OnClosePrintPreviewDialog();
 }
 
 void PrintPreviewUI::OnClosePrintPreviewDialog() {
   if (dialog_closed_)
     return;
   dialog_closed_ = true;
   ConstrainedWebDialogDelegate* delegate = GetConstrainedDelegate();
   if (!delegate)
     return;
@@ skipping 22 matching lines @@
 }
 
 void PrintPreviewUI::SetSelectedFileForTesting(const base::FilePath& path) {
   handler_->FileSelected(path, 0, NULL);
 }
 
 void PrintPreviewUI::SetPdfSavedClosureForTesting(
     const base::Closure& closure) {
   handler_->SetPdfSavedClosureForTesting(closure);
 }