Disallow UI/IO thread blocking on any other thread.

base/threading/sequenced_worker_pool.cc

Index: base/threading/sequenced_worker_pool.cc
===================================================================
--- base/threading/sequenced_worker_pool.cc	(revision 133458)
+++ base/threading/sequenced_worker_pool.cc	(working copy)
@@ -23,6 +23,7 @@
 #include "base/synchronization/lock.h"
 #include "base/threading/platform_thread.h"
 #include "base/threading/simple_thread.h"
+#include "base/threading/thread_restrictions.h"
 #include "base/time.h"
 #include "base/tracked_objects.h"
 
@@ -625,6 +626,7 @@
   TimeTicks shutdown_wait_begin = TimeTicks::Now();
 
   {
+    base::ThreadRestrictions::ScopedAllowWait allow_wait;
     AutoLock lock(lock_);
     while (!CanShutdown())
       can_shutdown_cv_.Wait();