Add more out of line copy ctors for complex classes.

cc/trees/property_tree.cc

Index: cc/trees/property_tree.cc
diff --git a/cc/trees/property_tree.cc b/cc/trees/property_tree.cc
index 7eea35e1152329e819f1dcfafa0ca08607e98d1b..d3b6a7eb752be274bd37c2ed369b550d4ae82359 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -1311,6 +1311,8 @@ ScrollTree::ScrollTree()
     : currently_scrolling_node_id_(-1),
       layer_id_to_scroll_offset_map_(ScrollTree::ScrollOffsetMap()) {}
 
+ScrollTree::ScrollTree(const ScrollTree& other) = default;
+
 ScrollTree::~ScrollTree() {}
 
 ScrollTree& ScrollTree::operator=(const ScrollTree& from) {
@@ -1720,6 +1722,8 @@ PropertyTrees::PropertyTrees()
   scroll_tree.SetPropertyTrees(this);
 }
 
+PropertyTrees::PropertyTrees(const PropertyTrees& other) = default;
+
 PropertyTrees::~PropertyTrees() {}
 
 bool PropertyTrees::operator==(const PropertyTrees& other) const {