Add support for (de)serializing cc::Layer hierarchy.

cc/layers/layer_proto_converter.cc

+// Copyright 2015 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_proto_converter.h"
+
+#include "base/stl_util.h"
+#include "cc/layers/layer.h"
+#include "cc/proto/layer.pb.h"
+#include "cc/trees/layer_tree_host_common.h"
+#include "cc/trees/layer_tree_settings.h"
+
+namespace cc {
+
+LayerProtoConverter::LayerProtoConverter() {}
+
+LayerProtoConverter::~LayerProtoConverter() {}
+
+// static
+void LayerProtoConverter::SerializeLayerHierarchy(
+    const scoped_refptr<Layer> root_layer,
+    proto::LayerNode* root_node) {
+  root_layer->ToLayerNodeProto(root_node);
+}
+
+// static
+scoped_refptr<Layer> LayerProtoConverter::DeserializeLayerHierarchy(
+    scoped_refptr<Layer> existing_root,
+    const proto::LayerNode& root_node) {
+  LayerIdMap layer_id_map;
+  RecursivelyFindAllLayers(existing_root, &layer_id_map);
+
+  scoped_refptr<Layer> new_root = existing_root;
+  if (!existing_root ||
+      (root_node.has_id() && root_node.id() != existing_root->id())) {
+    // The root node has changed or there was no root node,
+    // so find or create the new root.
+    new_root = FindOrAllocateAndConstruct(root_node, layer_id_map);
+  }
+  new_root->FromLayerNodeProto(root_node, layer_id_map);
+  return new_root;
+}
+
+// static
+void LayerProtoConverter::RecursivelyFindAllLayers(
+    const scoped_refptr<Layer>& layer,
+    LayerIdMap* layer_id_map) {
+  LayerTreeHostCommon::CallFunctionForSubtree(
+      layer.get(),
+      [layer_id_map](Layer* layer) { (*layer_id_map)[layer->id()] = layer; });
+}
+
+// static
+scoped_refptr<Layer> LayerProtoConverter::FindOrAllocateAndConstruct(
+    const proto::LayerNode& proto,
+    const Layer::LayerIdMap& layer_id_map) {
+  DCHECK(proto.has_id());
+  Layer::LayerIdMap::const_iterator iter = layer_id_map.find(proto.id());
+  if (iter != layer_id_map.end())
+    return iter->second;
+  DCHECK(proto.has_type());
+  switch (proto.type()) {
+    case proto::Base:

[vmpstr, 2015/10/23 21:26:10]

Do other cases exist already? If there's only Base for now, I'd prefer you drop
the default case, so that when new enums are added it won't compile until the
author updates this switch. 

[nyquist, 2015/10/26 03:14:29]

Done.

For now it's only base (until the TODO is implemented). Unsure about the style
you prefer for this though. Added a NOTREACHED() below the switch for now and
returning nullptr to make it compile. Is that OK? Happy to change to whatever
format you want!

[vmpstr, 2015/10/26 17:51:45]

Yep, that's perfect.

+      return Layer::Create(LayerSettings()).get();
+    default:
+      // TODO(nyquist): Add the rest of the necessary LayerTypes. This function
+      // should not return null.
+      return nullptr;
+  }
+}
+
+}  // namespace cc