Enforce singleton not allowed behavior for CONTINUE_ON_SHUTDOWN tasks

CONTINUE_ON_SHUTDOWN means the thread running the task will not be
joined on shutdown and may not have a valid singleton/lazy_instance
at that moment.

Also made DhcpcsvcInitSingleton explicitly leaky to avoid
the AssertSingletonAllowed() dcheck.

BUG=478209

Committed: https://crrev.com/de7e9038663d69356e558764d5a7b215e8b49b5b
Review-Url: https://codereview.chromium.org/1904273002
Cr-Original-Commit-Position: refs/heads/master@{#390125}
Cr-Commit-Position: refs/heads/master@{#406053}

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.

net/proxy/dhcpcsvc_init_win.cc

@@ -19,15 +19,12 @@ class DhcpcsvcInitSingleton {
     DWORD err = DhcpCApiInitialize(&version);
     DCHECK(err == ERROR_SUCCESS);  // DCHECK_EQ complains of unsigned mismatch.
   }
-
-  ~DhcpcsvcInitSingleton() {
-    // Worker pool threads that use the DHCP API may still be running, so skip
-    // cleanup.
-  }
 };
 
-static base::LazyInstance<DhcpcsvcInitSingleton> g_dhcpcsvc_init_singleton =
-    LAZY_INSTANCE_INITIALIZER;
+// Worker pool threads that use the DHCP API may still be running at shutdown.
+// Leak instance and skip cleanup.
+static base::LazyInstance<DhcpcsvcInitSingleton>::Leaky
+    g_dhcpcsvc_init_singleton = LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace