Wire up Android back button in SkyShell

sky/engine/public/platform/WebInputEvent.h

Index: sky/engine/public/platform/WebInputEvent.h
diff --git a/sky/engine/public/platform/WebInputEvent.h b/sky/engine/public/platform/WebInputEvent.h
index b63becb1ef8b27bf9f1afec0e741a19257ef01a8..d3e58ed78d8d07fbd2a6cbe353c4f3e6b1d4fda0 100644
--- a/sky/engine/public/platform/WebInputEvent.h
+++ b/sky/engine/public/platform/WebInputEvent.h
@@ -123,6 +123,8 @@ public:
         GestureTypeLast = GesturePinchUpdate,
 
         WheelEvent,
+
+        Back,
     };
 
     enum Modifiers {
@@ -187,6 +189,10 @@ public:
         return type == WheelEvent;
     }
 
+    static WebInputEvent* create() {
+        return new WebInputEvent(sizeof(WebInputEvent));
+    }
+
 protected:
     explicit WebInputEvent(unsigned sizeParam)
     {