Add logging code to help diagnose random failure

layout/base/nsDocumentViewer.cpp

@@ -3968,7 +3968,10 @@ DocumentViewerImpl::Cancel()
 NS_IMETHODIMP
 DocumentViewerImpl::ExitPrintPreview()
 {
-  if (GetIsPrinting()) return NS_ERROR_FAILURE;
+  printf("TEST-INFO ExitPrintPreview: mPrintEngine=%p, GetIsPrinting()=%d\n",
+         mPrintEngine.get(), GetIsPrinting());
+  if (GetIsPrinting())
+    return NS_ERROR_FAILURE;
   NS_ENSURE_TRUE(mPrintEngine, NS_ERROR_FAILURE);
 
   if (GetIsPrintPreview()) {