Add Block shutdown on MachineLevelUserCloudPolicyStore Task runner

This makes sure the update policy cache operation is always finished.
This flag is used by other cloud policy store also:
https://cs.chromium.org/chromium/src/chrome/browser/profiles/profile_impl.cc?type=cs&q=ProfileImpl+Traits&g=0&l=353

Change-Id: Ief21485c12b4a6b666aa8e7ecccb96e4e9540724
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1582769
Commit-Queue: Owen Min <zmin@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654533}

chrome/browser/policy/machine_level_user_cloud_policy_controller.cc

@@ -135,7 +135,10 @@ MachineLevelUserCloudPolicyController::CreatePolicyManager(
       MachineLevelUserCloudPolicyStore::Create(
           dm_token, client_id, policy_dir, cloud_policy_has_priority,
           base::CreateSequencedTaskRunnerWithTraits(
-              {base::MayBlock(), base::TaskPriority::BEST_EFFORT}));
+              {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
+               // Block shutdown to make sure the policy cache update is always
+               // finished.
+               base::TaskShutdownBehavior::BLOCK_SHUTDOWN}));
   return std::make_unique<MachineLevelUserCloudPolicyManager>(
       std::move(policy_store), nullptr, policy_dir,
       base::ThreadTaskRunnerHandle::Get(),