base: Remove a copy of and copy ctor from SequenceAndSortKey.

base/task_scheduler/scheduler_thread_pool.cc

Index: base/task_scheduler/scheduler_thread_pool.cc
diff --git a/base/task_scheduler/scheduler_thread_pool.cc b/base/task_scheduler/scheduler_thread_pool.cc
index b00990bb82945e8026f5cb5ad263462e39277863..f7e24b11aa75dba4e6624123f1c50a3d33ce4b45 100644
--- a/base/task_scheduler/scheduler_thread_pool.cc
+++ b/base/task_scheduler/scheduler_thread_pool.cc
@@ -238,7 +238,7 @@ SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::GetWork(
     SchedulerWorkerThread* worker_thread) {
   std::unique_ptr<PriorityQueue::Transaction> transaction(
       outer_->shared_priority_queue_.BeginTransaction());
-  const auto sequence_and_sort_key = transaction->Peek();
+  const auto& sequence_and_sort_key = transaction->Peek();
 
   if (sequence_and_sort_key.is_null()) {
     // |transaction| is kept alive while |worker_thread| is added to