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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
electron::util::Promise<gin_helper::Dictionary> promise) {
    open_promise_ =
        std::make_unique<electron::util::Promise<gin_helper::Dictionary>>(
            std::move(promise));
    RunAsynchronous();
  }
  base::FilePath GetFileName() const {
    gchar* filename = gtk_file_chooser_get_filename(GTK_FILE_CHOOSER(dialog_));
    const base::FilePath path(filename);
    g_free(filename);
    return path;
  }
  std::vector<base::FilePath> GetFileNames() const {
    std::vector<base::FilePath> paths;
    auto* filenames = gtk_file_chooser_get_filenames(GTK_FILE_CHOOSER(dialog_));
    for (auto* iter = filenames; iter != nullptr; iter = iter->next) {
      auto* filename = static_cast<char*>(iter->data);
      paths.emplace_back(filename);
      g_free(filename);
    }
    g_slist_free(filenames);
    return paths;
  }
  CHROMEG_CALLBACK_1(FileChooserDialog,
                     void,
                     OnFileDialogResponse,
                     GtkWidget*,
                     int);
  GtkWidget* dialog() const { return dialog_; }