MD Settings: Convert all browser proxies to use ES6 class syntax.

chrome/browser/resources/settings/downloads_page/downloads_browser_proxy.js

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 cr.define('settings', function() {
   /** @interface */
-  function DownloadsBrowserProxy() {}
-
-  DownloadsBrowserProxy.prototype = {
-    initializeDownloads: assertNotReached,
-
-    selectDownloadLocation: assertNotReached,
-
-    resetAutoOpenFileTypes: assertNotReached,
-  };
+  class DownloadsBrowserProxy {
+    initializeDownloads() {}
+    selectDownloadLocation() {}
+    resetAutoOpenFileTypes() {}
+  }
 
   /**
    * @implements {settings.DownloadsBrowserProxy}
-   * @constructor
    */
-  function DownloadsBrowserProxyImpl() {}
+  class DownloadsBrowserProxyImpl {
+    /** @override */
+    initializeDownloads() {
+      chrome.send('initializeDownloads');
+    }
+
+    /** @override */
+    selectDownloadLocation() {
+      chrome.send('selectDownloadLocation');
+    }
+
+    /** @override */
+    resetAutoOpenFileTypes() {
+      chrome.send('resetAutoOpenFileTypes');
+    }
+  }
 
   cr.addSingletonGetter(DownloadsBrowserProxyImpl);
 
-  DownloadsBrowserProxyImpl.prototype = {
-    /** @override */
-    initializeDownloads: function() {
-      chrome.send('initializeDownloads');
-    },
-
-    /** @override */
-    selectDownloadLocation: function() {
-      chrome.send('selectDownloadLocation');
-    },
-
-    /** @override */
-    resetAutoOpenFileTypes: function() {
-      chrome.send('resetAutoOpenFileTypes');
-    },
-  };
-
   return {
     DownloadsBrowserProxy: DownloadsBrowserProxy,
     DownloadsBrowserProxyImpl: DownloadsBrowserProxyImpl,
   };
 });