fix: update GetPlugins to GetPluginsAsync for API rename

Upstream renamed PluginService methods:
- GetPlugins() -> GetPluginsAsync() (async version takes callback)
- GetPluginsSynchronous() -> GetPlugins() (sync version returns directly)

Electron uses the async version, so updated the call from GetPlugins()
to GetPluginsAsync().

Ref: https://chromium-review.googlesource.com/c/chromium/src/+/7139361

shell/browser/electron_plugin_info_host_impl.cc

@@ -36,7 +36,7 @@ void ElectronPluginInfoHostImpl::GetPluginInfo(const GURL& url,
                                                GetPluginInfoCallback callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   GetPluginInfo_Params params = {url, origin, mime_type};
-  PluginService::GetInstance()->GetPlugins(
+  PluginService::GetInstance()->GetPluginsAsync(
       base::BindOnce(&ElectronPluginInfoHostImpl::PluginsLoaded,
                      weak_factory_.GetWeakPtr(), params, std::move(callback)));
 }