Reset the sequence checker after UnsafeResetForTesting()

In unit tests, it turns out that the AtomicFlag may still be stuck to
its previous sequence. Detach it in UnsafeResetForTesting to avoid that.

Change-Id: I34b2e4953424c5aebd6ec55c8b13086617a16098
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089759Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747327}

base/synchronization/atomic_flag.cc

@@ -24,6 +24,7 @@ void AtomicFlag::Set() {
 }
 
 void AtomicFlag::UnsafeResetForTesting() {
+  DETACH_FROM_SEQUENCE(set_sequence_checker_);
   flag_.store(0, std::memory_order_release);
 }