[ThreadPool]: Reduce lock scope in ThreadGroupImpl::Start

DCHECK is triggered in base::internal::CheckedLockImpl::Acquire from:
base::FeatureList::IsFeatureEnabled(base::Feature const &)
base::FeatureList::IsEnabled(base::Feature const &)
base::internal::ThreadGroupImpl::Start

This is probably related to MayBlockWithoutDelay feature recently
enabled on canary.

Fix: Move lock's scope to exclude IsEnabled.


Bug: 1027333
Change-Id: I7663ee109a4c26b0ca500760394748da2572802d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929865Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718632}

base/task/thread_pool/thread_group_impl.cc

@@ -387,12 +387,6 @@ void ThreadGroupImpl::Start(
     Optional<TimeDelta> may_block_threshold) {
   DCHECK(!replacement_thread_group_);
 
-  ScopedWorkersExecutor executor(this);
-
-  CheckedAutoLock auto_lock(lock_);
-
-  DCHECK(workers_.empty());
-
   in_start().may_block_without_delay =
       FeatureList::IsEnabled(kMayBlockWithoutDelay) &&
       priority_hint_ == ThreadPriority::NORMAL;
@@ -405,6 +399,10 @@ void ThreadGroupImpl::Start(
       priority_hint_ == ThreadPriority::NORMAL ? kForegroundBlockedWorkersPoll
                                                : kBackgroundBlockedWorkersPoll;
 
+  ScopedWorkersExecutor executor(this);
+  CheckedAutoLock auto_lock(lock_);
+
+  DCHECK(workers_.empty());
   max_tasks_ = max_tasks;
   DCHECK_GE(max_tasks_, 1U);
   in_start().initial_max_tasks = max_tasks_;