Clear UKM Event aggregates on Purge/Rotate

Change-Id: Ia189dd201e5cd504e8190d88da20a8aab541a450
Reviewed-on: https://chromium-review.googlesource.com/944184Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540299}

components/ukm/ukm_recorder_impl.cc

@@ -156,6 +156,7 @@ void UkmRecorderImpl::Purge() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   sources_.clear();
   entries_.clear();
+  event_aggregations_.clear();
 }
 
 void UkmRecorderImpl::SetIsWebstoreExtensionCallback(
@@ -230,6 +231,7 @@ void UkmRecorderImpl::StoreRecordingsInReport(Report* report) {
                             unsent_sources.size());
   sources_.clear();
   entries_.clear();
+  event_aggregations_.clear();
 
   // Keep at most |max_kept_sources|, prioritizing most-recent entries (by
   // creation time).