Allow empty UKM entries to be recorded.

components/ukm/ukm_entry_builder.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/ukm/ukm_entry_builder.h"
 
 #include "base/metrics/metrics_hashes.h"
 #include "components/ukm/ukm_entry.h"
 #include "components/ukm/ukm_service.h"
 
 namespace ukm {
 
 UkmEntryBuilder::UkmEntryBuilder(const UkmService::AddEntryCallback& callback,
                                  int32_t source_id,
                                  const char* event_name)
     : add_entry_callback_(callback),
       entry_(new UkmEntry(source_id, event_name)) {}
 
 UkmEntryBuilder::~UkmEntryBuilder() {
-  if (entry_->metrics_.empty())
-    return;
-
   add_entry_callback_.Run(std::move(entry_));
 }
 
 void UkmEntryBuilder::AddMetric(const char* metric_name, int64_t value) {
   entry_->metrics_.emplace_back(
       std::make_pair(base::HashMetricName(metric_name), value));
 }
 
 }  // namespace ukm