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

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 2fec36364570707340f3d6bc57fb886b51e901ee..3a6ebf6e1cb843baf5ba6029c518e40931e5dae4 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -67,6 +67,10 @@ class ScrollbarLayerInterface;
 class SimpleEnclosedRegion;
 struct AnimationEvent;
 
+namespace proto {
+class LayerNode;
+}  // proto
+
 // Base class for composited layers. Special layer types are derived from
 // this class.
 class CC_EXPORT Layer : public base::RefCounted<Layer>,
@@ -74,6 +78,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer>,
                         public LayerAnimationValueProvider {
  public:
   typedef LayerList LayerListType;
+  typedef base::hash_map<int, scoped_refptr<Layer>> LayerIdMap;
 
   enum LayerIdLabels {
     INVALID_ID = -1,
@@ -358,6 +363,18 @@ class CC_EXPORT Layer : public base::RefCounted<Layer>,
 
   virtual void PushPropertiesTo(LayerImpl* layer);
 
+  // Recursively iterate over this layer and all children and write the
+  // hierarchical structure to the given LayerNode proto. In addition to the
+  // structure itself, the Layer id and type is also written to facilitate
+  // construction of the correct layer on the client.
+  virtual void ToLayerNodeProto(proto::LayerNode* proto) const;
+  // Recursively iterate over the given LayerNode proto and read the structure

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

nit: can you put a blank line before this one?

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

Done.

+  // into this node and its children. The |layer_map| should be used to look
+  // for previously existing Layers, since they should be re-used between each
+  // hierarchy update.
+  virtual void FromLayerNodeProto(const proto::LayerNode& proto,
+                                  const LayerIdMap& layer_map);
+
   LayerTreeHost* layer_tree_host() { return layer_tree_host_; }
   const LayerTreeHost* layer_tree_host() const { return layer_tree_host_; }