Bug 962881 - Fix regression to y scrolling with x overflow:hidden. r=botond

--HG--
extra : rebase_source : 32cc1c9f4abdb8d9f32319d4b718637e93691155

gfx/layers/ipc/AsyncPanZoomController.cpp

@@ -1228,7 +1228,7 @@ bool FlingAnimation::Sample(FrameMetrics& aFrameMetrics,
     mX.AdjustDisplacement(cssOffset.x, overscroll.x,
                           aFrameMetrics.GetDisableScrollingX()),
     mY.AdjustDisplacement(cssOffset.y, overscroll.y,
-                          aFrameMetrics.GetDisableScrollingX())
+                          aFrameMetrics.GetDisableScrollingY())
   ));
 
   return true;