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

cc/proto/layer.proto

+// 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.
+
+syntax = "proto2";
+
+package cc.proto;
+
+option optimize_for = LITE_RUNTIME;
+
+// Identifies the type of cc:Layer a LayerNode represents. It is used to
+// facilitate reconstruction of a Layer of the correct type on the client.
+enum LayerType {
+  Base = 1;
+
+  // TODO(nyquist): Add the rest of the necessary LayerTypes.
+};
+
+// Hierarchical structure for serializing the Layer tree.
+message LayerNode {
+  // required
+  optional int32 id = 1;
+  // required
+  optional LayerType type = 2;
+  optional int32 parent_id = 3;
+  // A List of all the children of the current LayerNode.
+  repeated LayerNode children = 4;
+  optional LayerNode mask_layer = 5;
+  optional LayerNode replica_layer = 6;
+}