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 9369fb7adea535ea262a41ffe3850f133eddcb97..a85eb4d3d84ce8486244275c2fc0599c860cb0af 100644
--- a/base/threading/sequenced_worker_pool.h
+++ b/base/threading/sequenced_worker_pool.h
@@ -45,7 +45,8 @@ class SequencedTaskRunner;
 //     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,
@@ -161,9 +162,12 @@ class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
   // an unsequenced task, returns an invalid SequenceToken.
   static SequenceToken GetSequenceTokenForCurrentThread();
 
-  // Returns the SequencedWorkerPool that owns this thread, or null if the
-  // current thread is not a SequencedWorkerPool worker thread.
-  static scoped_refptr<SequencedWorkerPool> GetWorkerPoolForCurrentThread();
+  // 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();
 
   // When constructing a SequencedWorkerPool, there must be a
   // ThreadTaskRunnerHandle on the current thread unless you plan to
@@ -182,7 +186,7 @@ class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
 
   // Returns a unique token that can be used to sequence tasks posted to
   // PostSequencedWorkerTask(). Valid tokens are always nonzero.
-  SequenceToken GetSequenceToken();
+  static SequenceToken GetSequenceToken();
 
   // Returns the sequence token associated with the given name. Calling this
   // function multiple times with the same string will always produce the