Made layout viewport scroll updates from compositor work like ordinary layers.

cc/trees/layer_tree_host_common.cc

Index: cc/trees/layer_tree_host_common.cc
diff --git a/cc/trees/layer_tree_host_common.cc b/cc/trees/layer_tree_host_common.cc
index 0263d5537c286c66985dc5c18281a7356d6deafc..9c65531f5ae2292889d9aa673ae5ec5b947af16c 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -140,6 +140,9 @@ LayerTreeHostCommon::CalcDrawPropsImplInputsForTesting::
                                         gfx::Transform(),
                                         render_surface_layer_list) {}
 
+LayerTreeHostCommon::ScrollUpdateInfo::ScrollUpdateInfo()
+    : layer_id(Layer::INVALID_ID) {}
+
 bool LayerTreeHostCommon::ScrollUpdateInfo::operator==(
     const LayerTreeHostCommon::ScrollUpdateInfo& other) const {
   return layer_id == other.layer_id && scroll_delta == other.scroll_delta;