Added pinch viewport offset to history item.

Source/core/frame/PinchViewport.h

 /*
  * Copyright (C) 2013 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 63 matching lines @@
 
     // Sets the location of the inner viewport relative to the outer viewport. The
     // coordinates are in partial CSS pixels.
     void setLocation(const FloatPoint&);
 
     // Sets the size of the inner viewport when unscaled in CSS pixels.
     // This will be clamped to the size of the outer viewport (the main frame).
     void setSize(const IntSize&);
     IntSize size() const { return m_size; }
 
+    // Resets the viewport to initial state.
+    void reset();
+
     // Let the viewport know that the main frame changed size (either through screen
     // rotation on Android or window resize elsewhere).
     void mainFrameDidChangeSize();
 
     void setScale(float);
     float scale() const { return m_scale; }
 
     void registerLayersWithTreeView(blink::WebLayerTreeView*) const;
     void clearLayersForTreeView(blink::WebLayerTreeView*) const;
 
@@ skipping 44 matching lines @@
 
     // Offset of the pinch viewport from the main frame's origin, in CSS pixels.
     FloatPoint m_offset;
     float m_scale;
     IntSize m_size;
 };
 
 } // namespace WebCore
 
 #endif // PinchViewport_h