cc: Stop creating unused 0 property tree nodes other than transform

cc/trees/property_tree.h

 // Copyright 2014 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_TREES_PROPERTY_TREE_H_
 #define CC_TREES_PROPERTY_TREE_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 49 matching lines @@
   PropertyTree();
   PropertyTree(const PropertyTree& other) = delete;
 
   // These C++ special member functions cannot be implicit inline because
   // they are exported by CC_EXPORT. They will be instantiated in every
   // compilation units that included this header, and compilation can fail
   // because T may be incomplete.
   ~PropertyTree();
   PropertyTree<T>& operator=(const PropertyTree<T>&);
 
+  // Property tree node starts from index 0.
+  static const int kInvalidNodeId = -1;
+  static const int kRootNodeId = 0;
+
   bool operator==(const PropertyTree<T>& other) const;
 
   int Insert(const T& tree_node, int parent_id);
 
   T* Node(int i) {
     // TODO(vollick): remove this.
     CHECK(i < static_cast<int>(nodes_.size()));
     return i > -1 ? &nodes_[i] : nullptr;
   }
   const T* Node(int i) const {
     // TODO(vollick): remove this.
     CHECK(i < static_cast<int>(nodes_.size()));
     return i > -1 ? &nodes_[i] : nullptr;
   }
 
-  T* parent(const T* t) { return Node(t->parent_id); }
-  const T* parent(const T* t) const { return Node(t->parent_id); }
+  T* parent(const T* t) {
+    return t->parent_id >= 0 ? Node(t->parent_id) : nullptr;
+  }
+  const T* parent(const T* t) const {
+    return t->parent_id >= 0 ? Node(t->parent_id) : nullptr;
+  }
 
   T* back() { return size() ? &nodes_.back() : nullptr; }
   const T* back() const { return size() ? &nodes_.back() : nullptr; }
 
   void clear();
   size_t size() const { return nodes_.size(); }
 
   void set_needs_update(bool needs_update) { needs_update_ = needs_update; }
   bool needs_update() const { return needs_update_; }
 
@@ skipping 25 matching lines @@
   TransformTree();
 
   // These C++ special member functions cannot be implicit inline because
   // they are exported by CC_EXPORT. They will be instantiated in every
   // compilation units that included this header, and compilation can fail
   // because TransformCachedNodeData may be incomplete.
   TransformTree(const TransformTree&) = delete;
   ~TransformTree();
   TransformTree& operator=(const TransformTree&);
 
+  // Transform tree requires a dummy node 0 representing device space. The root
+  // node of transform tree with real information is node 1, unlike all other
+  // property trees. When transform tree is created, it is created with dummy
+  // node 0.
+  static const int kDeviceNodeId = 0;
+  static const int kRootNodeId = 1;
+
   bool operator==(const TransformTree& other) const;
 
   int Insert(const TransformNode& tree_node, int parent_id);
 
   void clear();
 
   // Computes the change of basis transform from node |source_id| to |dest_id|.
   // The function returns false iff the inverse of a singular transform was
   // used (and the result should, therefore, not be trusted). Transforms may
   // be computed between any pair of nodes that have an ancestor/descendant
@@ skipping 160 matching lines @@
   float page_scale_factor_;
   float device_scale_factor_;
   float device_transform_scale_factor_;
   std::vector<int> nodes_affected_by_inner_viewport_bounds_delta_;
   std::vector<int> nodes_affected_by_outer_viewport_bounds_delta_;
   std::vector<TransformCachedNodeData> cached_data_;
 };
 
 class CC_EXPORT ClipTree final : public PropertyTree<ClipNode> {
  public:
+  static const int kViewportNodeId = 0;
+
   bool operator==(const ClipTree& other) const;
 
   void SetViewportClip(gfx::RectF viewport_rect);
   gfx::RectF ViewportClip();
 
   void ToProtobuf(proto::PropertyTree* proto) const;
   void FromProtobuf(const proto::PropertyTree& proto,
                     std::unordered_map<int, int>* node_id_to_index_map);
 };
 
@@ skipping 246 matching lines @@
   gfx::Vector2dF inner_viewport_container_bounds_delta_;
   gfx::Vector2dF outer_viewport_container_bounds_delta_;
   gfx::Vector2dF inner_viewport_scroll_bounds_delta_;
 
   PropertyTreesCachedData cached_data_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_PROPERTY_TREE_H_