Just account for context disposal garbage collections in context disposal stats counters.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 5167b6a45b7be6fdf2579f4ea7456d2ccc5b1230..ddc12be50dac327a66c90ac0bfc1234f56d5b109 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -4465,8 +4465,8 @@ bool Heap::IdleNotification(int idle_time_in_ms) {
       break;
     }
     case DO_FULL_GC: {
-      HistogramTimerScope scope(isolate_->counters()->gc_context());
       if (contexts_disposed_) {
+        HistogramTimerScope scope(isolate_->counters()->gc_context());
         CollectAllGarbage(kNoGCFlags, "idle notification: contexts disposed");
         gc_idle_time_handler_.NotifyIdleMarkCompact();
         gc_count_at_last_idle_gc_ = gc_count_;