Initial draft - modify ViewportAnchor to know about both inner and outer viewports.

Source/web/WebViewImpl.h

 /*
  * Copyright (C) 2010 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 472 matching lines @@
 
     bool pinchVirtualViewportEnabled() const;
 
     bool matchesHeuristicsForGpuRasterizationForTesting() const { return m_matchesHeuristicsForGpuRasterization; }
 
 private:
     // TODO(bokan): Remains for legacy pinch. Remove once it's gone. Made private to
     // prevent external usage
     virtual void setPageScaleFactor(float scaleFactor, const WebPoint& origin) OVERRIDE;
 
+    void scrollAndRescaleViewports(float scaleFactor, const IntPoint& mainFrameOrigin, const FloatPoint& pinchViewportOrigin);
+
+    IntRect visibleRectInDocument() const;
+
     float legibleScale() const;
     void refreshPageScaleFactorAfterLayout();
     void resumeTreeViewCommits();
     void setUserAgentPageScaleConstraints(PageScaleConstraints newConstraints);
     float clampPageScaleFactorToLimits(float) const;
     IntSize contentsSize() const;
 
     void resetSavedScrollAndScaleState();
 
     void updateMainFrameScrollPosition(const IntPoint& scrollPosition, bool programmaticScroll);
@@ skipping 212 matching lines @@
     bool m_userGestureObserved;
 };
 
 // We have no ways to check if the specified WebView is an instance of
 // WebViewImpl because WebViewImpl is the only implementation of WebView.
 DEFINE_TYPE_CASTS(WebViewImpl, WebView, webView, true, true);
 
 } // namespace blink
 
 #endif