Wire up Android back button in SkyShell

sky/engine/public/sky/sky_view.cc

Index: sky/engine/public/sky/sky_view.cc
diff --git a/sky/engine/public/sky/sky_view.cc b/sky/engine/public/sky/sky_view.cc
index 2050150b1b11cf5b7e38edd2192925cd1991b6d3..d16243f6e7dd8a7323ee5f11f09a53a550d42450 100644
--- a/sky/engine/public/sky/sky_view.cc
+++ b/sky/engine/public/sky/sky_view.cc
@@ -91,6 +91,8 @@ void SkyView::HandleInputEvent(const WebInputEvent& inputEvent) {
   } else if (WebInputEvent::isWheelEventType(inputEvent.type)) {
       const WebWheelEvent& event = static_cast<const WebWheelEvent&>(inputEvent);
       data_->view_->handleInputEvent(WheelEvent::create(event));
+  } else if (inputEvent.type == WebInputEvent::Back) {
+      data_->view_->handleInputEvent(Event::create("back"));
   }
 
 }