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

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 b934131c2f5a7c872596a5efa21e9a4168851200..fc6ae2fea5e582366b6278a0d639e34e3e1fac69 100644
--- a/Source/core/frame/PinchViewport.h
+++ b/Source/core/frame/PinchViewport.h
@@ -80,6 +80,7 @@ public:
     // coordinates are in partial CSS pixels.
     void setLocation(const FloatPoint&);
     void move(const FloatPoint&);
+    FloatPoint location() const { return m_offset; } // used in unit tests
 
     // 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).