[heap] Fix a formatting bug in --trace-incremental-marking.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index ad8d1466b92d8d6c43167914f395c6a8dc7394a2..a57e0454123398f91a09ae21e64e52d590d9a6b4 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -810,7 +810,9 @@ void Heap::ScheduleIdleScavengeIfNeeded(int bytes_allocated) {
 
 void Heap::FinalizeIncrementalMarking(GarbageCollectionReason gc_reason) {
   if (FLAG_trace_incremental_marking) {
-    isolate()->PrintWithTimestamp("[IncrementalMarking] (%s).\n", gc_reason);
+    isolate()->PrintWithTimestamp(
+        "[IncrementalMarking] (%s).\n",
+        Heap::GarbageCollectionReasonToString(gc_reason));
   }
 
   HistogramTimerScope incremental_marking_scope(