Fix Brave's Welcome page

The welcome page was not loading due to the following JS error:

  Uncaught ReferenceError: PromiseResolver is not defined
      at Object.sendWithPromise (cr.js:344)
      at welcomeUtils.ts:16
      at Object.dispatch (index.js:8)
      at Object.getSearchEngineProviders (redux.js:462)
      at loadWelcomeData (brave_welcome.tsx:26)
      at HTMLDocument.initialize (brave_welcome.tsx:31)

As it was also done already for brave_new_tab.html (search for
the "Fixes NTP" commit), we fix this error by doing this:

  - Removed import of cr.html as import has been deprecated
  - Added sourcing of promise_resolver.js to brave_welcome.html

components/brave_welcome_ui/brave_welcome.html

@@ -6,7 +6,7 @@
 <title>Welcome to Brave</title>
 <link rel="stylesheet" href="chrome://resources/css/text_defaults.css">
 <link rel="shortcut icon" href="favicon.ico" type="image/x-icon">
-<link rel="import" href="chrome://resources/html/cr.html">
+<script src="chrome://resources/js/promise_resolver.js"></script>
 <script src="chrome://resources/js/cr.js"></script>
 <script src="chrome://resources/js/load_time_data.js"></script>
 <script src="chrome://resources/js/util.js"></script>