WebUI: prevent WebContent to hold an invalid pointer.

chrome/browser/ui/webui/print_preview/print_preview_ui.cc

Index: chrome/browser/ui/webui/print_preview/print_preview_ui.cc
diff --git a/chrome/browser/ui/webui/print_preview/print_preview_ui.cc b/chrome/browser/ui/webui/print_preview/print_preview_ui.cc
index 9c7a8f80956fe6b94b2e282a002f934e2af67f11..6898f7f40dc1eeac21060dba82470ae173e6f458 100644
--- a/chrome/browser/ui/webui/print_preview/print_preview_ui.cc
+++ b/chrome/browser/ui/webui/print_preview/print_preview_ui.cc
@@ -654,8 +654,11 @@ void PrintPreviewUI::OnHidePreviewDialog() {
   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();
 }