Revert of Revert "Allow SequencedTaskRunnerHandle::Get() while running unsequenced tasks."

base/threading/sequenced_task_runner_handle.cc

Index: base/threading/sequenced_task_runner_handle.cc
diff --git a/base/threading/sequenced_task_runner_handle.cc b/base/threading/sequenced_task_runner_handle.cc
index db96a6ada942741d6b3b8b96946c56680340afdd..88b36a8d648694cf892816389dc49ff2e15454c0 100644
--- a/base/threading/sequenced_task_runner_handle.cc
+++ b/base/threading/sequenced_task_runner_handle.cc
@@ -28,20 +28,16 @@
   if (handle) {
     // Various modes of setting SequencedTaskRunnerHandle don't combine.
     DCHECK(!base::ThreadTaskRunnerHandle::IsSet());
-    DCHECK(!SequencedWorkerPool::GetSequenceTokenForCurrentThread().IsValid());
+    DCHECK(!SequencedWorkerPool::GetSequencedTaskRunnerForCurrentThread());
     return handle->task_runner_;
   }
 
-  // If we are on a worker thread for a SequencedBlockingPool that is running a
-  // sequenced task, return a SequencedTaskRunner for it.
-  scoped_refptr<base::SequencedWorkerPool> pool =
-      SequencedWorkerPool::GetWorkerPoolForCurrentThread();
-  if (pool) {
-    SequencedWorkerPool::SequenceToken sequence_token =
-        SequencedWorkerPool::GetSequenceTokenForCurrentThread();
-    DCHECK(sequence_token.IsValid());
-    DCHECK(pool->IsRunningSequenceOnCurrentThread(sequence_token));
-    return pool->GetSequencedTaskRunner(sequence_token);
+  // Return the SequencedTaskRunner obtained from SequencedWorkerPool, if any.
+  scoped_refptr<base::SequencedTaskRunner> task_runner =
+      SequencedWorkerPool::GetSequencedTaskRunnerForCurrentThread();
+  if (task_runner) {
+    DCHECK(!base::ThreadTaskRunnerHandle::IsSet());
+    return task_runner;
   }
 
   // Return the SingleThreadTaskRunner for the current thread otherwise.
@@ -51,7 +47,7 @@
 // static
 bool SequencedTaskRunnerHandle::IsSet() {
   return lazy_tls_ptr.Pointer()->Get() ||
-         SequencedWorkerPool::GetSequenceTokenForCurrentThread().IsValid() ||
+         SequencedWorkerPool::GetWorkerPoolForCurrentThread() ||
          base::ThreadTaskRunnerHandle::IsSet();
 }