Enforce singleton not allowed behavior for CONTINUE_ON_SHUTDOWN tasks

base/threading/sequenced_worker_pool.cc

Index: base/threading/sequenced_worker_pool.cc
diff --git a/base/threading/sequenced_worker_pool.cc b/base/threading/sequenced_worker_pool.cc
index 57961b5cd5538306ea00a4b354966e32b20d4e0c..9002755d21319feae01a3773f96951faf9b4e3e5 100644
--- a/base/threading/sequenced_worker_pool.cc
+++ b/base/threading/sequenced_worker_pool.cc
@@ -247,6 +247,14 @@ class SequencedWorkerPool::Worker : public SimpleThread {
     is_processing_task_ = true;
     task_sequence_token_ = token;
     task_shutdown_behavior_ = shutdown_behavior;
+
+    // It is dangerous for tasks with CONTINUE_ON_SHUTDOWN to access a class
+    // that implements a non-leaky base::Singleton because they are generally
+    // destroyed before the process terminates via an AtExitManager
+    // registration. This will trigger a DCHECK to warn of such cases. See the
+    // comment about CONTINUE_ON_SHUTDOWN for more details.
+    ThreadRestrictions::SetSingletonAllowed(task_shutdown_behavior_ !=
+                                            CONTINUE_ON_SHUTDOWN);
   }
 
   // Indicates that the task has finished running.