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

base/threading/sequenced_worker_pool.h

Index: base/threading/sequenced_worker_pool.h
diff --git a/base/threading/sequenced_worker_pool.h b/base/threading/sequenced_worker_pool.h
index 4d90f920830c87fb2166b9cdd961a05c49783de0..b45fad33ecc8be8973c674029d9b13ce7490128a 100644
--- a/base/threading/sequenced_worker_pool.h
+++ b/base/threading/sequenced_worker_pool.h
@@ -48,7 +48,8 @@
 //     destruction will be visible to T2.
 //
 // Example:
-//   SequencedWorkerPool::SequenceToken token = pool.GetSequenceToken();
+//   SequencedWorkerPool::SequenceToken token =
+//       SequencedWorkerPool::GetSequenceToken();
 //   pool.PostSequencedWorkerTask(token, SequencedWorkerPool::SKIP_ON_SHUTDOWN,
 //                                FROM_HERE, base::Bind(...));
 //   pool.PostSequencedWorkerTask(token, SequencedWorkerPool::SKIP_ON_SHUTDOWN,
@@ -164,13 +165,22 @@
   // an unsequenced task, returns an invalid SequenceToken.
   static SequenceToken GetSequenceTokenForCurrentThread();
 
+  // Gets a SequencedTaskRunner for the current thread. If the current thread is
+  // running an unsequenced task, a new SequenceToken will be generated and set,
+  // so that the returned SequencedTaskRunner is guaranteed to run tasks after
+  // the current task has finished running.
+  static scoped_refptr<SequencedTaskRunner>
+  GetSequencedTaskRunnerForCurrentThread();
+
+  // Returns a unique token that can be used to sequence tasks posted to
+  // PostSequencedWorkerTask(). Valid tokens are always nonzero.
+  // TODO(bauerb): Rename this to better differentiate from
+  // GetSequenceTokenForCurrentThread().
+  static SequenceToken GetSequenceToken();
+
   // Returns the SequencedWorkerPool that owns this thread, or null if the
   // current thread is not a SequencedWorkerPool worker thread.
   static scoped_refptr<SequencedWorkerPool> GetWorkerPoolForCurrentThread();
-
-  // Returns a unique token that can be used to sequence tasks posted to
-  // PostSequencedWorkerTask(). Valid tokens are always nonzero.
-  static SequenceToken GetSequenceToken();
 
   // When constructing a SequencedWorkerPool, there must be a
   // ThreadTaskRunnerHandle on the current thread unless you plan to
@@ -323,6 +333,10 @@
   // sequence_token.
   bool IsRunningSequenceOnCurrentThread(SequenceToken sequence_token) const;
 
+  // Returns true if any thread is currently processing a task with the given
+  // sequence token. Should only be called with a valid sequence token.
+  bool IsRunningSequence(SequenceToken sequence_token) const;
+
   // Blocks until all pending tasks are complete. This should only be called in
   // unit tests when you want to validate something that should have happened.
   // This will not flush delayed tasks; delayed tasks get deleted.