Switch BaseSessionService to use SequencedWorkerPool instead of FILE thread

chrome/browser/sessions/base_session_service.h

Index: chrome/browser/sessions/base_session_service.h
diff --git a/chrome/browser/sessions/base_session_service.h b/chrome/browser/sessions/base_session_service.h
index be2c258dc3d3ed81ea4648165c3e375a7538a6bb..fc6eaa5cefb0c82f49e7d3be6a30a58184de70ad 100644
--- a/chrome/browser/sessions/base_session_service.h
+++ b/chrome/browser/sessions/base_session_service.h
@@ -13,6 +13,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "chrome/browser/common/cancelable_request.h"
 #include "chrome/browser/sessions/session_id.h"
 #include "chrome/common/cancelable_task_tracker.h"
@@ -154,14 +155,15 @@ class BaseSessionService : public CancelableRequestProvider {
       const InternalGetCommandsCallback& callback,
       CancelableTaskTracker* tracker);
 
-  // In production, this posts the task to the FILE thread.  For
+  // In production, this posts the task to the SequencedWorkerPool.  For
   // tests, it immediately runs the specified task on the current
   // thread.
   bool RunTaskOnBackendThread(const tracked_objects::Location& from_here,
                               const base::Closure& task);
 
   // Returns true if we appear to be running in production, false if we appear
-  // to be running as part of a unit test or if the FILE thread has gone away.
+  // to be running as part of a unit test or if the SequencedWorkerPool has
+  // been asked to shutdown.
   bool RunningInProduction() const;
 
   // Max number of navigation entries in each direction we'll persist.
@@ -189,6 +191,9 @@ class BaseSessionService : public CancelableRequestProvider {
   // The number of commands sent to the backend before doing a reset.
   int commands_since_reset_;
 
+  // A token to make sure that all tasks will be serialized.
+  base::SequencedWorkerPool::SequenceToken sequence_token_;
+
   DISALLOW_COPY_AND_ASSIGN(BaseSessionService);
 };