Wire up Android back button in SkyShell

sky/services/viewport/input_event.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module sky;
 
 enum EventType {
   UNKNOWN,
   POINTER_CANCEL,
   POINTER_DOWN,
   POINTER_MOVE,
   POINTER_UP,
   GESTURE_FLING_CANCEL,
   GESTURE_FLING_START,
   GESTURE_LONG_PRESS,
   GESTURE_SCROLL_BEGIN,
   GESTURE_SCROLL_END,
   GESTURE_SCROLL_UPDATE,
   GESTURE_SHOW_PRESS,
   GESTURE_TAP,
   GESTURE_TAP_DOWN,
+  BACK,
 };
 
 enum PointerKind {
   TOUCH,
 };
 
 struct PointerData {
   int32 pointer;
   PointerKind kind;
   float x;
@@ skipping 22 matching lines @@
   float velocityX;
   float velocityY;
 };
 
 struct InputEvent {
   EventType type;
   int64 time_stamp;
   PointerData? pointer_data;
   GestureData? gesture_data;
 };