Fix sky shell.

This add the missing RequestNavigateHistory method on sky::Engine.

It is currently not implemented.

TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/957973003

shell/ui/engine.cc

@@ -159,5 +159,9 @@ void Engine::RequestNavigate(mojo::Target target,
 void Engine::DidNavigateLocally(const mojo::String& url) {
 }
 
+void Engine::RequestNavigateHistory(int32_t delta) {
+  NOTIMPLEMENTED();
+}
+
 }  // namespace shell
 }  // namespace sky

shell/ui/engine.h

@@ -73,6 +73,7 @@ class Engine : public UIDelegate,
   void RequestNavigate(mojo::Target target,
                        mojo::URLRequestPtr request) override;
   void DidNavigateLocally(const mojo::String& url) override;
+  void RequestNavigateHistory(int32_t delta) override;
 
   void UpdateWebViewSize();