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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
std::string* file_extension) {
  
  if (*file_extension == ".*")
    return true;
  return base::EndsWith(file_info->filename, *file_extension,
                        base::CompareCase::INSENSITIVE_ASCII);
}
void OnFileFilterDataDestroyed(std::string* file_extension) {
  delete file_extension;
}
class FileChooserDialog {
 public:
  FileChooserDialog(GtkFileChooserAction action, const DialogSettings& settings)
      : parent_(
            static_cast<electron::NativeWindowViews*>(settings.parent_window)),
        filters_(settings.filters) {
    const char* confirm_text = gtk_util::kOkLabel;
    if (!settings.button_label.empty())
      confirm_text = settings.button_label.c_str();
    else if (action == GTK_FILE_CHOOSER_ACTION_SAVE)
      confirm_text = gtk_util::kOpenLabel;
    else if (action == GTK_FILE_CHOOSER_ACTION_OPEN)
      confirm_text = gtk_util::kSaveLabel;
    dialog_ = gtk_file_chooser_dialog_new(
        settings.title.c_str(), nullptr, action, gtk_util::kCancelLabel,
        GTK_RESPONSE_CANCEL, confirm_text, GTK_RESPONSE_ACCEPT, NULL);
    if (parent_) {
      parent_->SetEnabled(false);
      libgtkui::SetGtkTransientForAura(dialog_, parent_->GetNativeWindow());