Use swipe gesture to switch between slides on Android

tools/viewer/sk_app/Window.h

 /*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #ifndef Window_DEFINED
 #define Window_DEFINED
 
@@ skipping 112 matching lines @@
 
     bool onChar(SkUnichar c, uint32_t modifiers);
     bool onKey(Key key, InputState state, uint32_t modifiers);
     bool onMouse(int x, int y, InputState state, uint32_t modifiers);
     void onPaint();
     void onResize(uint32_t width, uint32_t height);
 
     uint32_t width() { return fWidth; }
     uint32_t height() { return fHeight;  }
 
-    const DisplayParams& getDisplayParams();
+    virtual const DisplayParams& getDisplayParams();
     void setDisplayParams(const DisplayParams& params);
 
 protected:
     Window();
 
     uint32_t     fWidth;
     uint32_t     fHeight;
 
     OnCharFunc   fCharFunc;
     void*        fCharUserData;
     OnKeyFunc    fKeyFunc;
     void*        fKeyUserData;
     OnMouseFunc  fMouseFunc;
     void*        fMouseUserData;
     OnPaintFunc  fPaintFunc;
     void*        fPaintUserData;
 
     WindowContext* fWindowContext;
 };
 
 }   // namespace sk_app
 #endif