Loosen/improve AtomicFlag semantics.

base/synchronization/atomic_flag.cc

Index: base/synchronization/atomic_flag.cc
diff --git a/base/synchronization/atomic_flag.cc b/base/synchronization/atomic_flag.cc
index 98d90cb88fd6c039842eb6ac5ccd6bd6017abf9f..8c2018d369324bb4ea64b8cf4ccabe989a3eab71 100644
--- a/base/synchronization/atomic_flag.cc
+++ b/base/synchronization/atomic_flag.cc
@@ -8,10 +8,16 @@
 
 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);
 }