Cancel the pending print preview request when the user request to show the system dialog.

chrome/browser/ui/webui/print_preview_ui.h

Index: chrome/browser/ui/webui/print_preview_ui.h
diff --git a/chrome/browser/ui/webui/print_preview_ui.h b/chrome/browser/ui/webui/print_preview_ui.h
index ccd2e77f6eb8f0e0994c91698153377bdf396bbc..73f7c3e51d8132e0dba1269d88490b991d37f86c 100644
--- a/chrome/browser/ui/webui/print_preview_ui.h
+++ b/chrome/browser/ui/webui/print_preview_ui.h
@@ -96,6 +96,9 @@ class PrintPreviewUI : public ChromeWebUI {
   // Notifies the Web UI renderer that file selection has been cancelled.
   void OnFileSelectionCancelled();
 
+  // Notifies the Web UI to cancel the pending preview request.
+  void OnCancelPendingPreviewRequest();
+
  private:
   FRIEND_TEST_ALL_PREFIXES(PrintPreviewTabControllerUnitTest,
                            TitleAfterReload);