Add support for deserializing without existing root node

When deserializing a layer tree for the first time, there is no
existing root. Not all parts of the code supported this scenario,
and this CL adds support for this use-case for the hierarchy
deserialization.

They property deserialization still requires an existing root
since the deserialization should have happened before.

This also adds a test for the initial deserialization.

BUG=577310
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1584263002

Cr-Commit-Position: refs/heads/master@{#369636}

3 files changed, 36 insertions, 3 deletions

diff --git a/cc/layers/layer_proto_converter.cc b/cc/layers/layer_proto_converter.cc
index 6564e0b..086ad99 100644
--- a/cc/layers/layer_proto_converter.cc
+++ b/cc/layers/layer_proto_converter.cc
@@ -31,7 +31,8 @@ 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);
+  if (existing_root)
+    RecursivelyFindAllLayers(existing_root, &layer_id_map);
 
   scoped_refptr<Layer> new_root = existing_root;
   if (!existing_root ||
@@ -55,6 +56,7 @@ void LayerProtoConverter::SerializeLayerProperties(
 void LayerProtoConverter::DeserializeLayerProperties(
     Layer* existing_root,
     const proto::LayerUpdate& layer_update) {
+  DCHECK(existing_root);
   LayerIdMap layer_id_map;
   RecursivelyFindAllLayers(existing_root, &layer_id_map);
 
diff --git a/cc/layers/layer_proto_converter.h b/cc/layers/layer_proto_converter.h
index 7f2742b..f821a5d 100644
--- a/cc/layers/layer_proto_converter.h
+++ b/cc/layers/layer_proto_converter.h
@@ -27,7 +27,8 @@ class CC_EXPORT LayerProtoConverter {
   // 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|).
+  // |existing_root|). |existing_root| may be null, which might happen during
+  // the first deserialize.
   static scoped_refptr<Layer> DeserializeLayerHierarchy(
       const scoped_refptr<Layer> existing_root,
       const proto::LayerNode& root_node);
@@ -40,7 +41,8 @@ class CC_EXPORT LayerProtoConverter {
                                        proto::LayerUpdate* layer_update);
 
   // Iterate over all updated layers from the LayerUpdate, and update the
-  // local Layers.
+  // local Layers. |existing_root| must not be null, as that will make it
+  // impossible to find the layer to apply the properties to.
   static void DeserializeLayerProperties(
       Layer* existing_root,
       const proto::LayerUpdate& layer_update);
diff --git a/cc/layers/layer_proto_converter_unittest.cc b/cc/layers/layer_proto_converter_unittest.cc
index e1a30e6..3aa74d3 100644
--- a/cc/layers/layer_proto_converter_unittest.cc
+++ b/cc/layers/layer_proto_converter_unittest.cc
@@ -86,6 +86,35 @@ TEST_F(LayerProtoConverterTest, TestKeepingRoot) {
   EXPECT_EQ(child_c_node->id(), child_c->id());
 }
 
+TEST_F(LayerProtoConverterTest, TestNoExistingRoot) {
+  /* Test deserialization of a tree that looks like:
+         root
+        /
+       a
+     There is no existing root node before serialization.
+  */
+  int new_root_id = 244;
+  proto::LayerNode root_node;
+  root_node.set_id(new_root_id);
+  root_node.set_type(proto::LayerType::LAYER);
+
+  proto::LayerNode* child_a_node = root_node.add_children();
+  child_a_node->set_id(442);
+  child_a_node->set_type(proto::LayerType::LAYER);
+  child_a_node->set_parent_id(new_root_id);  // root_node
+
+  scoped_refptr<Layer> new_root =
+      LayerProtoConverter::DeserializeLayerHierarchy(nullptr, root_node);
+
+  // The new root should not be the same as the old root.
+  EXPECT_EQ(new_root_id, new_root->id());
+  ASSERT_EQ(1u, new_root->children().size());
+  scoped_refptr<Layer> child_a = new_root->children()[0];
+
+  EXPECT_EQ(child_a_node->id(), child_a->id());
+  EXPECT_EQ(0u, child_a->children().size());
+}
+
 TEST_F(LayerProtoConverterTest, TestSwappingRoot) {
   /* Test deserialization of a tree that looks like:
          root