[heap] Introduce enum of garbage collection reasons.

src/heap/scavenge-job.cc

Index: src/heap/scavenge-job.cc
diff --git a/src/heap/scavenge-job.cc b/src/heap/scavenge-job.cc
index d89c9453c50bb19da5aca78cda23a544ce4ce139..66d4307b6cfe12e0ebac72f737024eca2f1e2ccf 100644
--- a/src/heap/scavenge-job.cc
+++ b/src/heap/scavenge-job.cc
@@ -34,7 +34,7 @@ void ScavengeJob::IdleTask::RunInternal(double deadline_in_seconds) {
                                  new_space_capacity)) {
     if (EnoughIdleTimeForScavenge(
             idle_time_in_ms, scavenge_speed_in_bytes_per_ms, new_space_size)) {
-      heap->CollectGarbage(NEW_SPACE, "idle task: scavenge");
+      heap->CollectGarbage(NEW_SPACE, GarbageCollectionReason::kIdleTask);
     } else {
       // Immediately request another idle task that can get larger idle time.
       job_->RescheduleIdleTask(heap);