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

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 61a64d63402bb37d7c984c379b9cfa5e2b79d7a1..899c3cd5c1eb46617a87ff744cda017d2c179e2f 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -265,6 +265,8 @@ LayerImpl* LayerTreeImpl::CurrentlyScrollingLayer() const {
 }
 
 void LayerTreeImpl::SetCurrentlyScrollingLayer(LayerImpl* layer) {
+  DCHECK_IMPLIES(layer, layer != OuterViewportScrollLayer());
+
   int new_id = layer ? layer->id() : Layer::INVALID_ID;
   if (currently_scrolling_layer_id_ == new_id)
     return;