webshare: Small refactor of WebShare (OnceCallback, const&, and alias)

chrome/browser/ui/views/webshare/webshare_target_picker_view.h

Index: chrome/browser/ui/views/webshare/webshare_target_picker_view.h
diff --git a/chrome/browser/ui/views/webshare/webshare_target_picker_view.h b/chrome/browser/ui/views/webshare/webshare_target_picker_view.h
index 6e4db2ea5bc858c5ac6b87a1bca55ecab16c2cec..836f13cd9ccd8fadc51b5c19cfb7186f8d8957e7 100644
--- a/chrome/browser/ui/views/webshare/webshare_target_picker_view.h
+++ b/chrome/browser/ui/views/webshare/webshare_target_picker_view.h
@@ -38,8 +38,7 @@ class WebShareTargetPickerView : public views::DialogDelegateView,
   // the share.
   WebShareTargetPickerView(
       const std::vector<std::pair<base::string16, GURL>>& targets,
-      const base::Callback<void(base::Optional<std::string>)>&
-          close_callback);
+      chrome::WebShareTargetPickerCallback close_callback);
   ~WebShareTargetPickerView() override;
 
   // views::View overrides:
@@ -68,7 +67,7 @@ class WebShareTargetPickerView : public views::DialogDelegateView,
   const std::vector<std::pair<base::string16, GURL>> targets_;
   std::unique_ptr<TargetPickerTableModel> table_model_;
 
-  base::Callback<void(base::Optional<std::string>)> close_callback_;
+  chrome::WebShareTargetPickerCallback close_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(WebShareTargetPickerView);
 };