Loosen/improve AtomicFlag semantics.

base/synchronization/atomic_flag.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/synchronization/atomic_flag.h"
 
 #include "base/logging.h"
 
 namespace base {
 
-AtomicFlag::AtomicFlag() = default;
+AtomicFlag::AtomicFlag() {
+  // It doesn't matter where the AtomicFlag is built so long as it's always
+  // Set() from the same sequence after. Note: the sequencing requirements are
+  // necessary for IsSet()'s callers to know which sequence's memory operations
+  // they are synchronized with.
+  set_sequence_checker_.DetachFromSequence();
+}
 
 void AtomicFlag::Set() {
-  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(set_sequence_checker_.CalledOnValidSequence());
   base::subtle::Release_Store(&flag_, 1);
 }
 
 bool AtomicFlag::IsSet() const {
   return base::subtle::Acquire_Load(&flag_) != 0;
 }
 
 void AtomicFlag::UnsafeResetForTesting() {
   base::subtle::Release_Store(&flag_, 0);
 }
 
 }  // namespace base