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

cc/layers/layer_proto_converter.h

Index: cc/layers/layer_proto_converter.h
diff --git a/cc/layers/layer_proto_converter.h b/cc/layers/layer_proto_converter.h
new file mode 100644
index 0000000000000000000000000000000000000000..29424a0347f392a2407518f2af57fbcc3bf51440
--- /dev/null
+++ b/cc/layers/layer_proto_converter.h
@@ -0,0 +1,54 @@
+// 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.
+
+#ifndef CC_LAYERS_LAYER_PROTO_CONVERTER_H_
+#define CC_LAYERS_LAYER_PROTO_CONVERTER_H_
+
+#include "base/macros.h"
+#include "cc/base/cc_export.h"
+#include "cc/layers/layer.h"
+
+namespace cc {
+
+namespace proto {
+class LayerNode;
+}
+
+// A class to faciliate (de)serialization of a Layer tree to protocol buffers.
+class CC_EXPORT LayerProtoConverter {
+ public:
+  // Starting at |root_layer|, serializes the layer hierarchy into the
+  // proto::LayerNode.
+  static void SerializeLayerHierarchy(const scoped_refptr<Layer> root_layer,
+                                      proto::LayerNode* root_node);
+
+  // Recursively iterate over the given LayerNode proto and read the structure
+  // into a local Layer structure, re-using existing Layers. returns the new
+  // root Layer after updating the hierarchy (may be the same as
+  // |existing_root|).
+  static scoped_refptr<Layer> DeserializeLayerHierarchy(
+      scoped_refptr<Layer> existing_root,
+      const proto::LayerNode& root_node);
+
+  // Returns the Layer with proto.id() as the Layer id, if it exists in
+  // |layer_id_map|. Otherwise, a new Layer is constructed of the type given
+  // from proto.type().
+  static scoped_refptr<Layer> FindOrAllocateAndConstruct(
+      const proto::LayerNode& proto,
+      const Layer::LayerIdMap& layer_id_map);
+
+ private:
+  LayerProtoConverter();
+  ~LayerProtoConverter();
+
+  using LayerIdMap = base::hash_map<int, scoped_refptr<Layer>>;
+  // Start at |layer| and recursively add |layer| and all its children and
+  // special layers to |layer_id_map|.
+  static void RecursivelyFindAllLayers(const scoped_refptr<Layer>& layer,
+                                       LayerIdMap* layer_id_map);
+};
+
+}  // namespace cc
+
+#endif  // CC_LAYERS_LAYER_PROTO_CONVERTER_H_