Add support for scroll and fling gestures to 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_DOWN,
   POINTER_UP,
   POINTER_MOVE,
   POINTER_CANCEL,
+  GESTURE_SCROLL_BEGIN,
+  GESTURE_SCROLL_UPDATE,
+  GESTURE_SCROLL_END,
+  GESTURE_FLING_START,
+  GESTURE_FLING_CANCEL,
 };
 
 enum PointerKind {
   TOUCH,
 };
 
 struct PointerData {
   int32 pointer;
   PointerKind kind;
   float x;
   float y;
   int32 buttons;
   float pressure;
   float pressure_min;
   float pressure_max;
   float distance;
   float distance_min;
   float distance_max;
   float radius_major;
   float radius_minor;
   float radius_min;
   float radius_max;
   float orientation;
   float tilt;
 };
 
+struct GestureData {
+    float x;
+    float y;
+    float dx;
+    float dy;
+    float velocityX;
+    float velocityY;
+};
+
 struct InputEvent {
   EventType type;
   int64 time_stamp;
   PointerData? pointer_data;
+  GestureData? gesture_data;
 };