base: Re-apply WeakPtr support for SequencedWorkerPools, fixing deadlock

base/threading/sequenced_worker_pool.cc

Index: base/threading/sequenced_worker_pool.cc
diff --git a/base/threading/sequenced_worker_pool.cc b/base/threading/sequenced_worker_pool.cc
index 035ef2b872601a352aa697965fd5caf489f7ac1c..d9921689fe883de20873d64d48cbecc242e3aaae 100644
--- a/base/threading/sequenced_worker_pool.cc
+++ b/base/threading/sequenced_worker_pool.cc
@@ -1124,6 +1124,10 @@ SequencedWorkerPool::Inner::g_last_sequence_number_;
 // static
 SequencedWorkerPool::SequenceToken
 SequencedWorkerPool::GetSequenceTokenForCurrentThread() {
+  // Don't construct lazy instance on check.
+  if (g_lazy_tls_ptr == NULL)
+    return SequenceToken();
+
   SequencedWorkerPool::SequenceToken* token = g_lazy_tls_ptr.Get().Get();
   if (!token)
     return SequenceToken();