refactor: migrates util::Promise to gin (#20871)

* refactor: use gin in Promise

* refactor: separate Promise impl that returns nothing

* refactor: use Promise<void> for promise that returns nothing

* fix: methods should be able to run on both browser and renderer process

* fix: should not pass base::StringPiece across threads

* refactor: no more need to use different ResolvePromise for empty Promise

* refactor: move Promise to gin_helper

shell/browser/api/save_page_handler.cc

@@ -17,7 +17,7 @@ namespace electron {
 namespace api {
 
 SavePageHandler::SavePageHandler(content::WebContents* web_contents,
-                                 util::Promise<void*> promise)
+                                 gin_helper::Promise<void> promise)
     : web_contents_(web_contents), promise_(std::move(promise)) {}
 
 SavePageHandler::~SavePageHandler() = default;