Use the pinch viewport offset for tap disambiguation.

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..2a738a74e2fc1c976fa0661dcdbcbb1b578d54b6 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; }
 
     // 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).