Release Shared SchedulerWorkers After Joining

If shared SchedulerWorkers are released before joining, they are
effectively detached and may outlive the
SchedulerSingleThreadTaskRunnerManager.

BUG=735701

Change-Id: Ie83b50b99698a4059041454f579f4a2c1fbaabc0
Reviewed-on: https://chromium-review.googlesource.com/572233Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487197}

base/task_scheduler/scheduler_single_thread_task_runner_manager.cc

@@ -457,8 +457,6 @@ SchedulerSingleThreadTaskRunnerManager::CreateTaskRunnerWithTraitsImpl(
 }
 
 void SchedulerSingleThreadTaskRunnerManager::JoinForTesting() {
-  ReleaseSharedSchedulerWorkers();
-
   decltype(workers_) local_workers;
   {
     AutoSchedulerLock auto_lock(lock_);
@@ -474,6 +472,11 @@ void SchedulerSingleThreadTaskRunnerManager::JoinForTesting() {
         << "New worker(s) unexpectedly registered during join.";
     workers_ = std::move(local_workers);
   }
+
+  // Release shared SchedulerWorkers at the end so they get joined above. If
+  // this call happens before the joins, the SchedulerWorkers are effectively
+  // detached and may outlive the SchedulerSingleThreadTaskRunnerManager.
+  ReleaseSharedSchedulerWorkers();
 }
 
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