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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
for (const auto& filter : filters) {
    GtkFileFilter* gtk_filter = gtk_file_filter_new();
    for (const auto& extension : filter.second) {
      auto file_extension = std::make_unique<std::string>("." + extension);
      gtk_file_filter_add_custom(
          gtk_filter, GTK_FILE_FILTER_FILENAME,
          reinterpret_cast<GtkFileFilterFunc>(FileFilterCaseInsensitive),
          file_extension.release(),
          reinterpret_cast<GDestroyNotify>(OnFileFilterDataDestroyed));
    }
    gtk_file_filter_set_name(gtk_filter, filter.first.c_str());
    gtk_file_chooser_add_filter(GTK_FILE_CHOOSER(dialog_), gtk_filter);
  }
}
void FileChooserDialog::OnUpdatePreview(GtkWidget* chooser) {