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

cc/trees/layer_tree.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "cc/layers/layer.h"
+#include "cc/proto/layer_tree.pb.h"
+#include "cc/trees/layer_tree.h"
+
+namespace cc {
+
+LayerTree::LayerTree() {}
+
+LayerTree::~LayerTree() {}
+
+void LayerTree::AddLayerShouldPushProperties(Layer* layer) {
+  layers_that_should_push_properties_.insert(layer);
+}
+
+void LayerTree::RemoveLayerShouldPushProperties(Layer* layer) {
+  layers_that_should_push_properties_.erase(layer);
+}
+
+LayerSet& LayerTree::layers_that_should_push_properties() {
+  return layers_that_should_push_properties_;
+}
+
+void LayerTree::ToProtobuf(proto::LayerTree* proto) {
+  for (auto layer : layers_that_should_push_properties_) {
+    proto->add_layers_that_should_push_properties(layer->id());
+  }
+}
+
+void LayerTree::FromProtobuf(const proto::LayerTree& proto,
+                             LayerIdMap* layer_id_map) {
+  for (auto layer_id : proto.layers_that_should_push_properties()) {
+    AddLayerShouldPushProperties((*layer_id_map)[layer_id]);
+  }
+}
+
+}  // namespace cc