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

BUG=

Review URL: https://codereview.chromium.org/765773002

Cr-Commit-Position: refs/heads/master@{#25546}

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_;