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

Source/web/WebViewImpl.h

Index: Source/web/WebViewImpl.h
diff --git a/Source/web/WebViewImpl.h b/Source/web/WebViewImpl.h
index 9f27591f92a4b5343e24d5570ebad1cf61aa1f27..90d4ee86ded95a4deda8f77666cb9e0e72561ea0 100644
--- a/Source/web/WebViewImpl.h
+++ b/Source/web/WebViewImpl.h
@@ -498,6 +498,10 @@ private:
     // 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();