improve allocation accounting for incremental mark

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index 947c961af8993d6efc959d4d1c7765a1c2e352a1..bf69daea53e77868e51ceac26874544f6d4bf476 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -907,6 +907,8 @@ intptr_t IncrementalMarking::Step(intptr_t allocated_bytes,
                                   CompletionAction action,
                                   ForceMarkingAction marking,
                                   ForceCompletionAction completion) {
+  DCHECK(allocated_bytes >= 0);
+
   if (heap_->gc_state() != Heap::NOT_IN_GC || !FLAG_incremental_marking ||
       !FLAG_incremental_marking_steps ||
       (state_ != SWEEPING && state_ != MARKING)) {