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### Issue Details

* **Electron Version:**
  * 7.0.0
* **Operating System:**
  * Ubuntu 18.04 (KDE Neon)
* **Last Known Working Electron version:**
  * 6.0.12

### Expected Behavior
dialog.showOpenDialog() should have the default label "Open"
dialog.showSaveDialog() should have the default label "Save"

### Actual Behavior
The labels are erroneously switched.

### Screenshots
<!-- If applicable, add screenshots to help explain your problem. -->
![labels_wrong](https://user-images.githubusercontent.com/14349333/67726433-c03fb500-f9dd-11e9-8a8a-63626e7bde1f.png)

shell/browser/ui/file_dialog_gtk.cc
gchar* filename =
      gtk_file_chooser_get_preview_filename(GTK_FILE_CHOOSER(chooser));
  if (!filename) {
    gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(chooser),
                                               FALSE);
    return;
  }
  
  
  struct stat stat_buf;
  if (stat(filename, &stat_buf) != 0 || !S_ISREG(stat_buf.st_mode)) {
    g_free(filename);
    gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(chooser),
                                               FALSE);
    return;
  }
  
  GdkPixbuf* pixbuf = gdk_pixbuf_new_from_file_at_size(filename, kPreviewWidth,
                                                       kPreviewHeight, nullptr);
  g_free(filename);
  if (pixbuf) {
    gtk_image_set_from_pixbuf(GTK_IMAGE(preview_), pixbuf);
    g_object_unref(pixbuf);
  }
  gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(chooser),
                                             pixbuf ? TRUE : FALSE);
}
}  
bool ShowOpenDialogSync(const DialogSettings& settings,