Refactor SetViewportLayerIds to use a struct instead of 6 values

cc/test/test_layer_tree_host_base.cc

Index: cc/test/test_layer_tree_host_base.cc
diff --git a/cc/test/test_layer_tree_host_base.cc b/cc/test/test_layer_tree_host_base.cc
index 64fbfb01b87f9141e37ef3532a651cfcc9e3e653..fa791570357ba3278565357f1d7f0c043f077e06 100644
--- a/cc/test/test_layer_tree_host_base.cc
+++ b/cc/test/test_layer_tree_host_base.cc
@@ -132,10 +132,9 @@ void TestLayerTreeHostBase::SetupPendingTree(
   pending_layer->SetRasterSourceOnPending(raster_source, invalidation);
 
   pending_root->test_properties()->AddChild(std::move(pending_layer));
-  pending_tree->SetViewportLayersFromIds(
-      Layer::INVALID_ID, pending_tree->root_layer_for_testing()->id(),
-      Layer::INVALID_ID, Layer::INVALID_ID, Layer::INVALID_ID,
-      Layer::INVALID_ID);
+  LayerTreeImpl::ViewportLayerIds viewport_ids;
+  viewport_ids.page_scale = pending_tree->root_layer_for_testing()->id();
+  pending_tree->SetViewportLayersFromIds(viewport_ids);
 
   pending_layer_ = static_cast<FakePictureLayerImpl*>(
       host_impl()->pending_tree()->LayerById(id_));