Added pinch viewport offset to history item.

Source/core/frame/PinchViewport.h

Index: Source/core/frame/PinchViewport.h
diff --git a/Source/core/frame/PinchViewport.h b/Source/core/frame/PinchViewport.h
index d477e37ee0b1bff765038d229f5af21ef7e191da..ed0f01bccaaa1bbdb0a4d0a69d415687468b5f04 100644
--- a/Source/core/frame/PinchViewport.h
+++ b/Source/core/frame/PinchViewport.h
@@ -81,6 +81,9 @@ public:
     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();