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

BUG=

Review-Url: https://codereview.chromium.org/2324623004
Cr-Commit-Position: refs/heads/master@{#39278}

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(

src/isolate.h

@@ -1162,7 +1162,7 @@ class Isolate {
 
   bool IsIsolateInBackground() { return is_isolate_in_background_; }
 
-  void PrintWithTimestamp(const char* format, ...);
+  PRINTF_FORMAT(2, 3) void PrintWithTimestamp(const char* format, ...);
 
  protected:
   explicit Isolate(bool enable_serializer);