Fix shutdown crash in CertVerifier by using a MessageLoopProxy.

base/message_loop_proxy_impl.cc

Index: base/message_loop_proxy_impl.cc
diff --git a/base/message_loop_proxy_impl.cc b/base/message_loop_proxy_impl.cc
index 7998a9437ff4bbe138d7e8867594da0509ea72f6..c0619aa52c2c2922059325e0d23e85d8910168d1 100644
--- a/base/message_loop_proxy_impl.cc
+++ b/base/message_loop_proxy_impl.cc
@@ -78,6 +78,11 @@ bool MessageLoopProxyImpl::PostTaskHelper(
 }
 
 void MessageLoopProxyImpl::OnDestruct() const {
+  // We shouldn't use MessageLoop::current() since it uses LazyInstance which
+  // may be deleted by ~AtExitManager when a WorkerPool thread calls this
+  // function.
+  // http://crbug.com/63678
+  base::ThreadRestrictions::ScopedAllowSingleton allow_singleton;
   bool delete_later = false;
   {
     AutoLock lock(message_loop_lock_);