Scroll only visual viewport during OSX pinch gesture behind flag.

Source/core/frame/PinchViewport.cpp

 /*
  * Copyright (C) 2013 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 221 matching lines @@
         return false;
 
     // Keep the center-of-pinch anchor in a stable position over the course
     // of the magnify.
     FloatPoint anchorAtOldScale = anchor.scaledBy(1.f / oldPageScale);
     FloatPoint anchorAtNewScale = anchor.scaledBy(1.f / newPageScale);
     FloatSize anchorDelta = anchorAtOldScale - anchorAtNewScale;
 
     // First try to use the anchor's delta to scroll the FrameView.
     FloatSize anchorDeltaUnusedByScroll = anchorDelta;
-    FrameView* view = mainFrame()->view();
-    DoublePoint oldPosition = view->scrollPositionDouble();
-    view->scrollBy(DoubleSize(anchorDelta.width(), anchorDelta.height()), UserScroll);
-    DoublePoint newPosition = view->scrollPositionDouble();
-    anchorDeltaUnusedByScroll = anchorDelta - toFloatSize(newPosition - oldPosition);
+
+    if (!frameHost().settings().invertViewportScrollOrder()) {
+        FrameView* view = mainFrame()->view();
+        DoublePoint oldPosition = view->scrollPositionDouble();
+        view->scrollBy(DoubleSize(anchorDelta.width(), anchorDelta.height()), UserScroll);
+        DoublePoint newPosition = view->scrollPositionDouble();
+        anchorDeltaUnusedByScroll -= toFloatSize(newPosition - oldPosition);
+    }
 
     // Manually bubble any remaining anchor delta up to the pinch viewport.
     FloatPoint newLocation(location() + anchorDeltaUnusedByScroll);
     setScaleAndLocation(newPageScale, newLocation);
     return true;
 }
 
 // Modifies the top of the graphics layer tree to add layers needed to support
 // the inner/outer viewport fixed-position model for pinch zoom. When finished,
 // the tree will look like this (with * denoting added layers):
@@ skipping 393 matching lines @@
     } else if (graphicsLayer == m_rootTransformLayer) {
         name =  "Root Transform Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink