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Relaxed ordering is insufficient since the resulting task id is used by
the caller for order operations. Test case written by omerkatz@
demonstrates the race:

TEST_F(ConcurrentMarkingTest, JobTaskIds) {
  static constexpr size_t kNumTasksToRun = 2;
  static size_t tmp[kNumTasksToRun] = { 0 };
  base::JobHandle handle = base::PostJob(
      FROM_HERE, {base::ThreadPool(), base::TaskPriority::USER_VISIBLE},
      ConvertToBaseRepeatingCallback(WTF::CrossThreadBindRepeating(
          [](base::JobDelegate* job) {
            uint8_t id = job->GetTaskId();
            size_t& slot = tmp[id];
            slot++;
          })),
      ConvertToBaseRepeatingCallback(WTF::CrossThreadBindRepeating(
          [](size_t) { return kNumTasksToRun; })));
  handle.Join();
}

Thread 1 can release its task id and thread 2 immediately pick it up,
which causes a data race on tmp when both thread access the same idx
with no memory fence in between. Adding memory_order_seq_cst on
Release/AcquireTaskId ensures that different thread with same task id
are sequential.

Bug: 1119518
Change-Id: Ifa35ca03e3663f53f76ee830bcaa8654bc48838c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2387554Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804082}