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

cc/layers/layer_proto_factory.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_factory.h"
+
+#include "cc/layers/layer.h"
+#include "cc/proto/layer.pb.h"
+#include "cc/trees/layer_tree_settings.h"
+
+namespace cc {
+
+// static
+scoped_refptr<Layer> LayerProtoFactory::FindOrAllocateAndConstruct(
+    const proto::LayerNode& proto,
+    const Layer::LayerIdMap& layer_id_map) {
+  if (proto.has_id()) {

[vmpstr, 2015/10/21 22:34:14]

Can you DCHECK that it has id?

[nyquist, 2015/10/23 00:12:31]

Done.

+    Layer::LayerIdMap::const_iterator iter = layer_id_map.find(proto.id());
+    if (iter != layer_id_map.end())
+      return iter->second;
+  }
+  if (!proto.has_type())

[vmpstr, 2015/10/21 22:34:14]

When can this happen? I don't think we handle nullptr in the rest of the code

[nyquist, 2015/10/23 00:12:31]

Done.

+    return nullptr;
+  switch (proto.type()) {
+    case proto::Base:
+      return Layer::Create(LayerSettings()).get();
+    default:
+      // TODO(nyquist): Add the rest of the necessary LayerTypes.
+      return nullptr;
+  }
+}
+
+}  // namespace cc