Refactor RegisterViewportLayers to use a struct instead of 6 values

cc/trees/layer_tree_host_unittest_picture.cc

Index: cc/trees/layer_tree_host_unittest_picture.cc
diff --git a/cc/trees/layer_tree_host_unittest_picture.cc b/cc/trees/layer_tree_host_unittest_picture.cc
index 3d7c33b96e4539a749e55e5ae6c7c1e370514a59..e4fefef207ce86a7a6750dc8224f03ecdccffe52 100644
--- a/cc/trees/layer_tree_host_unittest_picture.cc
+++ b/cc/trees/layer_tree_host_unittest_picture.cc
@@ -423,8 +423,11 @@ class LayerTreeHostPictureTestRSLLMembershipWithScale
     picture_->SetBounds(gfx::Size(100, 100));
     pinch_->AddChild(picture_);
 
-    layer_tree_host()->RegisterViewportLayers(NULL, page_scale_layer, root_clip,
-                                              nullptr, pinch_, nullptr);
+    LayerTreeHost::ViewportLayers viewport_layers;
+    viewport_layers.page_scale = page_scale_layer;
+    viewport_layers.inner_viewport_container = root_clip;
+    viewport_layers.inner_viewport_scroll = pinch_;
+    layer_tree_host()->RegisterViewportLayers(viewport_layers);
     layer_tree_host()->SetPageScaleFactorAndLimits(1.f, 1.f, 4.f);
     layer_tree_host()->SetRootLayer(root_clip);
     LayerTreeHostPictureTest::SetupTree();