Aggregate UKM events with no metrics.

Bug: 932525
Change-Id: Idd0a434ec165ec9de5f4b4f4b4578fea6d1f797c
Reviewed-on: https://chromium-review.googlesource.com/c/1482565
Auto-Submit: Brian White <bcwhite@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636171}

components/ukm/ukm_recorder_impl.cc

@@ -322,9 +322,8 @@ void UkmRecorderImpl::StoreRecordingsInReport(Report* report) {
 
     serialized_source_type_counts[GetSourceIdType(kv.first)]++;
   }
+
   for (const auto& event_and_aggregate : recordings_.event_aggregations) {
-    if (event_and_aggregate.second.metrics.empty())
-      continue;
     const EventAggregate& event_aggregate = event_and_aggregate.second;
     Aggregate* proto_aggregate = report->add_aggregates();
     proto_aggregate->set_source_id(0);  // Across all sources.