Reset property tree indices when layer is removed from layer tree

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 209c8a98715c1c2065a8b8d9a923e6c2ca95d7da..83b33afd676925c29d40a7969819e254c97a9159 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -141,8 +141,16 @@ class CC_EXPORT PropertyTree {
 
   int Insert(const T& tree_node, int parent_id);
 
-  T* Node(int i) { return i > -1 ? &nodes_[i] : nullptr; }
-  const T* Node(int i) const { return i > -1 ? &nodes_[i] : nullptr; }
+  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); }
@@ -246,6 +254,7 @@ class CC_EXPORT PropertyTrees final {
   OpacityTree opacity_tree;
   ClipTree clip_tree;
   bool needs_rebuild;
+  int sequence_number;
 };
 
 }  // namespace cc