Setup LayerTree class, refactor 2 functions from LayerTreeHost to it.

cc/layers/layer_unittest.cc

Index: cc/layers/layer_unittest.cc
diff --git a/cc/layers/layer_unittest.cc b/cc/layers/layer_unittest.cc
index 179e0a50eff8e70f516d90a8e95b500a330eb2f7..6c2611f7eb881f01ed02082a9ef52e81b8a72b12 100644
--- a/cc/layers/layer_unittest.cc
+++ b/cc/layers/layer_unittest.cc
@@ -143,16 +143,17 @@ class LayerSerializationTest : public testing::Test {
 
     // The |dest| layer needs to be able to lookup the scroll and clip parents.
     if (src->inputs_.scroll_parent)
-      layer_tree_host_->RegisterLayer(src->inputs_.scroll_parent);
+      layer_tree_host_->GetLayerTree()->RegisterLayer(
+          src->inputs_.scroll_parent);
     if (src->scroll_children_) {
       for (auto* child : *(src->scroll_children_))
-        layer_tree_host_->RegisterLayer(child);
+        layer_tree_host_->GetLayerTree()->RegisterLayer(child);
     }
     if (src->inputs_.clip_parent)
-      layer_tree_host_->RegisterLayer(src->inputs_.clip_parent);
+      layer_tree_host_->GetLayerTree()->RegisterLayer(src->inputs_.clip_parent);
     if (src->clip_children_) {
       for (auto* child : *(src->clip_children_))
-        layer_tree_host_->RegisterLayer(child);
+        layer_tree_host_->GetLayerTree()->RegisterLayer(child);
     }
     // Reset the LayerTreeHost registration for the |src| layer so
     // it can be re-used for the |dest| layer.
@@ -250,24 +251,26 @@ class LayerSerializationTest : public testing::Test {
 
     // Cleanup scroll tree.
     if (src->inputs_.scroll_parent)
-      layer_tree_host_->UnregisterLayer(src->inputs_.scroll_parent);
+      layer_tree_host_->GetLayerTree()->UnregisterLayer(
+          src->inputs_.scroll_parent);
     src->inputs_.scroll_parent = nullptr;
     dest->inputs_.scroll_parent = nullptr;
     if (src->scroll_children_) {
       for (auto* child : *(src->scroll_children_))
-        layer_tree_host_->UnregisterLayer(child);
+        layer_tree_host_->GetLayerTree()->UnregisterLayer(child);
       src->scroll_children_.reset();
       dest->scroll_children_.reset();
     }
 
     // Cleanup clip tree.
     if (src->inputs_.clip_parent)
-      layer_tree_host_->UnregisterLayer(src->inputs_.clip_parent);
+      layer_tree_host_->GetLayerTree()->UnregisterLayer(
+          src->inputs_.clip_parent);
     src->inputs_.clip_parent = nullptr;
     dest->inputs_.clip_parent = nullptr;
     if (src->clip_children_) {
       for (auto* child : *(src->clip_children_))
-        layer_tree_host_->UnregisterLayer(child);
+        layer_tree_host_->GetLayerTree()->UnregisterLayer(child);
       src->clip_children_.reset();
       dest->clip_children_.reset();
     }