mandoline: Add back/forward support and UI.

mandoline/ui/phone_ui/phone_browser_application_delegate.cc

Index: mandoline/ui/phone_ui/phone_browser_application_delegate.cc
diff --git a/mandoline/ui/phone_ui/phone_browser_application_delegate.cc b/mandoline/ui/phone_ui/phone_browser_application_delegate.cc
index b43910cdba3c05e65a5e5600628b29e9d62db84f..93307aad9d51f0956e7db6bd9e79a584bb9fedbc 100644
--- a/mandoline/ui/phone_ui/phone_browser_application_delegate.cc
+++ b/mandoline/ui/phone_ui/phone_browser_application_delegate.cc
@@ -115,6 +115,13 @@ void PhoneBrowserApplicationDelegate::ProgressChanged(double progress) {
   // ...
 }
 
+
+void PhoneBrowserApplicationDelegate::BackForwardChanged(
+    web_view::mojom::ButtonState back_button,
+    web_view::mojom::ButtonState forward_button) {
+  // ...
+}
+
 void PhoneBrowserApplicationDelegate::TitleChanged(const mojo::String& title) {
   // ...
 }