Add more out of line copy ctors for complex classes.

net/cert/signed_tree_head.cc

Index: net/cert/signed_tree_head.cc
diff --git a/net/cert/signed_tree_head.cc b/net/cert/signed_tree_head.cc
index b430b3744a6b94e725775b6c575dedd9f017664c..8bf2b9f3ff7755fae4e6082f1f7528fbd166dafe 100644
--- a/net/cert/signed_tree_head.cc
+++ b/net/cert/signed_tree_head.cc
@@ -29,6 +29,8 @@ SignedTreeHead::SignedTreeHead(Version version,
   memcpy(this->sha256_root_hash, sha256_root_hash, kSthRootHashLength);
 }
 
+SignedTreeHead::SignedTreeHead(const SignedTreeHead& other) = default;
+
 SignedTreeHead::~SignedTreeHead() {}
 
 void PrintTo(const SignedTreeHead& sth, std::ostream* os) {