Add TODO for early return in PersistentHistogramStorage's dtor

Change-Id: I4406713382f182bf3441a9abdec2a5eae4f9d37e
Reviewed-on: https://chromium-review.googlesource.com/1013086
Commit-Queue: Xi Cheng <chengx@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550859}

base/metrics/persistent_histogram_storage.cc

@@ -39,6 +39,8 @@ PersistentHistogramStorage::~PersistentHistogramStorage() {
   PersistentHistogramAllocator* allocator = GlobalHistogramAllocator::Get();
   allocator->UpdateTrackingHistograms();
 
+  // TODO(chengx): Investigate making early return depend on whethere there are
+  // metrics to report at this point or not.
   if (disabled_)
     return;