Use correct task traits for mojo leveldb env background tasks.

components/leveldb/env_mojo.cc

Index: components/leveldb/env_mojo.cc
diff --git a/components/leveldb/env_mojo.cc b/components/leveldb/env_mojo.cc
index 4f82181a5a385ba03a875133adaef639aad84ad1..206b8d481da751d1b0a5bf415de9016444c02257 100644
--- a/components/leveldb/env_mojo.cc
+++ b/components/leveldb/env_mojo.cc
@@ -413,7 +413,9 @@ void MojoEnv::SleepForMicroseconds(int 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) {