mandoline: Add back/forward support and UI.

components/web_view/public/interfaces/web_view.mojom

 // Copyright 2015 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.
 
 module web_view.mojom;
 
 import "components/mus/public/interfaces/view_tree.mojom";
 import "network/public/interfaces/url_loader.mojom";
 
+enum ButtonState {
+  ENABLED,
+  DISABLED,
+};
+
 interface WebViewClient {
   // Page-generated request for a top level frame navigation.
   TopLevelNavigate(mojo.URLRequest request);
 
   // Loading and progress notifications.
   LoadingStateChanged(bool is_loading);
   ProgressChanged(double progress);
+  BackForwardChanged(ButtonState back_button, ButtonState forward_button);
 
   // TODO(beng): also forward text direction.
   TitleChanged(string? title);
 };
 
 interface WebView {
   // Navigate the top level frame to |request|.
   LoadRequest(mojo.URLRequest request);
 
   // Provide a ViewTreeClient for this specific WebView.
   GetViewTreeClient(mojo.ViewTreeClient& view_tree_client);
+
+  // Moves forward and backward.
+  GoBack();
+  GoForward();
 };
 
 interface WebViewFactory {
   CreateWebView(WebViewClient client, WebView& web_view);
 };