Refactor SetViewportLayerIds to use a struct instead of 6 values

cc/layers/layer_impl_unittest.cc

Index: cc/layers/layer_impl_unittest.cc
diff --git a/cc/layers/layer_impl_unittest.cc b/cc/layers/layer_impl_unittest.cc
index 8311773a4b556f005e74a8e5f5de8892ef5c02a5..07259ad1ab7433040c6fbfdd3933843d3bb0c641 100644
--- a/cc/layers/layer_impl_unittest.cc
+++ b/cc/layers/layer_impl_unittest.cc
@@ -140,9 +140,9 @@ TEST(LayerImplTest, VerifyLayerChangesAreTrackedProperly) {
 
   // Make root the inner viewport scroll layer. This ensures the later call to
   // |SetViewportBoundsDelta| will be on a viewport layer.
-  host_impl.active_tree()->SetViewportLayersFromIds(
-      Layer::INVALID_ID, Layer::INVALID_ID, Layer::INVALID_ID,
-      Layer::INVALID_ID, root->id(), Layer::INVALID_ID);
+  LayerTreeImpl::ViewportLayerIds viewport_ids;
+  viewport_ids.inner_viewport_scroll = root->id();
+  host_impl.active_tree()->SetViewportLayersFromIds(viewport_ids);
 
   root->test_properties()->force_render_surface = true;
   root->SetMasksToBounds(true);