[Nearby Connections] Flaky shutdown in SubmittableExecutor

- last_task_completed_ should only signal if it's already shutdown,
  otherwise last_task_completed_ will be stuck in insignalled state when
  the first task completes

Bug: 1116437
Change-Id: I8f2172a5af031c5073563b518d77adca29fa85bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352886
Commit-Queue: Alex Chau <alexchau@chromium.org>
Reviewed-by: Himanshu Jaju <himanshujaju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798141}

chrome/services/sharing/nearby/platform_v2/submittable_executor.cc

@@ -32,7 +32,7 @@ SubmittableExecutor::~SubmittableExecutor() {
   base::AutoLock al(lock_);
   DCHECK_EQ(num_incomplete_tasks_, 0);
 #endif  // DCHECK_IS_ON()
-}  // namespace chrome
+}
 
 // Once called, this method will prevent any future calls to Submit() or
 // Execute() from posting additional tasks. Previously posted asks will be
@@ -84,7 +84,7 @@ void SubmittableExecutor::RunTask(Runnable&& runnable) {
 
   base::AutoLock al(lock_);
   DCHECK_GE(num_incomplete_tasks_, 1);
-  if (--num_incomplete_tasks_ == 0)
+  if (--num_incomplete_tasks_ == 0 && is_shut_down_)
     last_task_completed_.Signal();
 }