[heap] Make AlwaysAllocateScope thread-safe.

src/heap/heap-inl.h

Index: src/heap/heap-inl.h
diff --git a/src/heap/heap-inl.h b/src/heap/heap-inl.h
index 29ba854846a5acd4dfa97b1afd1c54d1ddc920f2..36e56a0aba75e8e8e50b558d549885210c23d028 100644
--- a/src/heap/heap-inl.h
+++ b/src/heap/heap-inl.h
@@ -749,12 +749,12 @@ void Heap::SetSetterStubDeoptPCOffset(int pc_offset) {
 
 AlwaysAllocateScope::AlwaysAllocateScope(Isolate* isolate)
     : heap_(isolate->heap()), daf_(isolate) {
-  heap_->always_allocate_scope_depth_++;
+  heap_->always_allocate_scope_count_.Increment(1);
 }
 
 
 AlwaysAllocateScope::~AlwaysAllocateScope() {
-  heap_->always_allocate_scope_depth_--;
+  heap_->always_allocate_scope_count_.Increment(-1);
 }