mandoline: Add back/forward support and UI.

components/web_view/public/interfaces/web_view.mojom

Index: components/web_view/public/interfaces/web_view.mojom
diff --git a/components/web_view/public/interfaces/web_view.mojom b/components/web_view/public/interfaces/web_view.mojom
index 3eb490ee6d32513f4fc15254166e4772c8e47eef..8eff5c472b9f9048e00bcead5a13da663b7255f8 100644
--- a/components/web_view/public/interfaces/web_view.mojom
+++ b/components/web_view/public/interfaces/web_view.mojom
@@ -7,6 +7,11 @@ module web_view.mojom;
 import "network/public/interfaces/url_loader.mojom";
 import "components/view_manager/public/interfaces/view_tree.mojom";
 
+enum ButtonState {
+  ENABLED,
+  DISABLED,
+};
+
 interface WebViewClient {
   // Page-generated request for a top level frame navigation.
   TopLevelNavigate(mojo.URLRequest request);
@@ -14,6 +19,7 @@ interface WebViewClient {
   // 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);
@@ -25,6 +31,10 @@ interface WebView {
 
   // Provide a ViewTreeClient for this specific WebView.
   GetViewTreeClient(mojo.ViewTreeClient& view_tree_client);
+
+  // Moves forward and backward.
+  GoBack();
+  GoForward();
 };
 
 interface WebViewFactory {