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

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index b00d090a3d863eb43bbd2946455cf70dc4cc4cf9..7aa86d60bc9ba2033540f38655d0453079220591 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -67,6 +67,10 @@ class CC_EXPORT PropertyTree {
   ~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);
@@ -82,8 +86,12 @@ class CC_EXPORT PropertyTree {
     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; }
@@ -129,6 +137,13 @@ class CC_EXPORT TransformTree final : public PropertyTree<TransformNode> {
   ~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);
@@ -309,6 +324,8 @@ class CC_EXPORT TransformTree final : public PropertyTree<TransformNode> {
 
 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);