Ensure the viewport is always represented by the inner viewport layer.

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index cece54d151cbcdc3bf0a483ab2a35026469bada3..0ae62c71c067c7f9e6c4ac6e5a3c5088bf93cff5 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -969,6 +969,8 @@ void LayerTreeHost::RegisterViewportLayers(
     scoped_refptr<Layer> page_scale_layer,
     scoped_refptr<Layer> inner_viewport_scroll_layer,
     scoped_refptr<Layer> outer_viewport_scroll_layer) {
+  DCHECK_IMPLIES(inner_viewport_scroll_layer,
+                 inner_viewport_scroll_layer != outer_viewport_scroll_layer);
   overscroll_elasticity_layer_ = overscroll_elasticity_layer;
   page_scale_layer_ = page_scale_layer;
   inner_viewport_scroll_layer_ = inner_viewport_scroll_layer;