Wire up Android back button in SkyShell

sky/engine/public/platform/WebInputEvent.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 105 matching lines @@
         GestureDoubleTap,
         GestureTwoFingerTap,
         GestureLongPress,
         GestureLongTap,
         GesturePinchBegin,
         GesturePinchEnd,
         GesturePinchUpdate,
         GestureTypeLast = GesturePinchUpdate,
 
         WheelEvent,
+
+        Back,
     };
 
     enum Modifiers {
         // modifiers for all events:
         ShiftKey         = 1 << 0,
         ControlKey       = 1 << 1,
         AltKey           = 1 << 2,
         MetaKey          = 1 << 3,
 
         // modifiers for keyboard events:
@@ skipping 44 matching lines @@
     static bool isGestureEventType(int type)
     {
         return GestureTypeFirst <= type && type <= GestureTypeLast;
     }
 
     static bool isWheelEventType(int type)
     {
         return type == WheelEvent;
     }
 
+    static WebInputEvent* create() {
+        return new WebInputEvent(sizeof(WebInputEvent));
+    }
+
 protected:
     explicit WebInputEvent(unsigned sizeParam)
     {
         memset(this, 0, sizeParam);
         timeStampMS = 0.0;
         size = sizeParam;
         type = Undefined;
         modifiers = 0;
     }
 };
@@ skipping 130 matching lines @@
     {
         memset(&data, 0, sizeof(data));
     }
 };
 
 #pragma pack(pop)
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_PUBLIC_PLATFORM_WEBINPUTEVENT_H_