Dispatch touch events and perform default touch actions.

This is currently limited to simple single-touch actions. It does not include
momentum scrolling or pinch zooming.

components/compositing/headless.rs

@@ -120,6 +120,7 @@ impl CompositorEventListener for NullCompositor {
             Msg::ChangePageTitle(..) |
             Msg::ChangePageUrl(..) |
             Msg::KeyEvent(..) |
+            Msg::TouchEventProcessed(..) |
             Msg::SetCursor(..) |
             Msg::ViewportConstrained(..) => {}
             Msg::CreatePng(..) |