Hook up the SequencedWorkerPool to the browser thread.

base/threading/sequenced_worker_pool.h

Index: base/threading/sequenced_worker_pool.h
===================================================================
--- base/threading/sequenced_worker_pool.h	(revision 116082)
+++ base/threading/sequenced_worker_pool.h	(working copy)
@@ -93,6 +93,10 @@
     explicit SequenceToken() : id_(0) {}
     ~SequenceToken() {}
 
+    // Returns true if this is a null sequence token. The null sequence token
+    // has not been initialized.
+    bool is_null() const { return id_ == 0; }
+
     bool Equals(const SequenceToken& other) const {
       return id_ == other.id_;
     }