Use correct task traits for mojo leveldb env background tasks.

components/leveldb/env_mojo.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/leveldb/env_mojo.h"
 
 #include <errno.h>
 
 #include <memory>
 
@@ skipping 395 matching lines @@
 uint64_t MojoEnv::NowMicros() {
   return base::TimeTicks::Now().ToInternalValue();
 }
 
 void MojoEnv::SleepForMicroseconds(int micros) {
   // Round up to the next millisecond.
   base::PlatformThread::Sleep(base::TimeDelta::FromMicroseconds(micros));
 }
 
 void MojoEnv::Schedule(void (*function)(void* arg), void* arg) {
-  base::PostTask(FROM_HERE, base::Bind(function, arg));
+  base::PostTaskWithTraits(
+      FROM_HERE, base::TaskTraits().MayBlock().WithBaseSyncPrimitives(),

[michaeln, 2017/04/24 20:54:17]

lgtm - Do you know if these tasks are skipped whether other 'threads' will wait
indefinitely? I'm wondering if they should also be
WithShutdownBehavior(BLOCK_SHUTDOWN) for that potential reason?

[Marijn Kruisselbrink, 2017/04/24 21:05:23]

Good point, yes, I believe destructing the leveldb::DBImpl instance could block
indefinitely if a background task was scheduled, but then not run because of
SKIP_ON_SHUTDOWN. Added the correct shutdown behavior.

+      base::Bind(function, arg));
 }
 
 void MojoEnv::StartThread(void (*function)(void* arg), void* arg) {
   new Thread(function, arg);  // Will self-delete.
 }
 
 }  // namespace leveldb