base: Add out-of-line copy ctors for complex classes.

base/task_scheduler/priority_queue.cc

Index: base/task_scheduler/priority_queue.cc
diff --git a/base/task_scheduler/priority_queue.cc b/base/task_scheduler/priority_queue.cc
index 05a90b22f8d85d90bd958e8aa2de6247af4a86c0..2cd8980a4eefd5b4213bc50d6ef36281ec204d06 100644
--- a/base/task_scheduler/priority_queue.cc
+++ b/base/task_scheduler/priority_queue.cc
@@ -20,6 +20,9 @@ PriorityQueue::SequenceAndSortKey::SequenceAndSortKey(
     const SequenceSortKey& sort_key)
     : sequence(std::move(sequence)), sort_key(sort_key) {}
 
+PriorityQueue::SequenceAndSortKey::SequenceAndSortKey(
+    const SequenceAndSortKey& other) = default;
+
 PriorityQueue::SequenceAndSortKey::~SequenceAndSortKey() = default;
 
 PriorityQueue::Transaction::Transaction(PriorityQueue* outer_queue)