[heap] Make active_safepoint_scopes_ non-atomic

Field used to be atomic for AssertActive() but now that this method
uses another approach, we can make that field non-atomic again.

Bug: v8:11708
Change-Id: I5254aa5e655844739082144fc75ba9f0c13d1ba0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3317424Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78273}

src/heap/safepoint.cc

@@ -56,7 +56,7 @@ void IsolateSafepoint::EnterGlobalSafepointScope(Isolate* initiator) {
     IgnoreLocalGCRequests ignore_gc_requests(initiator->heap());
     LockMutex(initiator->main_thread_local_heap());
   }
-  CHECK_EQ(active_safepoint_scopes_.exchange(1), 0);
+  CHECK_EQ(++active_safepoint_scopes_, 1);
 
   barrier_.Arm();
 
@@ -106,7 +106,7 @@ void IsolateSafepoint::LockMutex(LocalHeap* local_heap) {
 
 void IsolateSafepoint::LeaveGlobalSafepointScope(Isolate* initiator) {
   local_heaps_mutex_.AssertHeld();
-  CHECK_EQ(active_safepoint_scopes_.exchange(0), 1);
+  CHECK_EQ(--active_safepoint_scopes_, 0);
   ClearSafepointRequestedFlags(IncludeMainThreadUnlessInitiator(initiator));
   barrier_.Disarm();
   local_heaps_mutex_.Unlock();

src/heap/safepoint.h

@@ -131,7 +131,7 @@ class IsolateSafepoint final {
   base::RecursiveMutex local_heaps_mutex_;
   LocalHeap* local_heaps_head_;
 
-  std::atomic<int> active_safepoint_scopes_;
+  int active_safepoint_scopes_;
 
   friend class Heap;
   friend class GlobalSafepoint;