Convert first contentful paint logging to the new UKM client API

components/ukm/ukm_entry.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.h"
 
 #include "base/metrics/metrics_hashes.h"
 #include "components/metrics/proto/ukm/entry.pb.h"
 #include "components/ukm/ukm_source.h"
 
 namespace ukm {
 
 UkmEntry::UkmEntry(const content::GlobalRequestID& source_id,
                    const char* event_name)
     : source_id_(source_id), event_hash_(base::HashMetricName(event_name)) {}
 
 UkmEntry::~UkmEntry() {}
 
-void UkmEntry::PopulateProto(Entry* proto_entry) {
+void UkmEntry::PopulateProto(Entry* proto_entry) const {
   DCHECK(!proto_entry->has_source_id());
   DCHECK(!proto_entry->has_event_hash());
   DCHECK(proto_entry->metrics_size() == 0);
 
   proto_entry->set_source_id(UkmSource::GetHashedSourceID(source_id_));
   proto_entry->set_event_hash(event_hash_);
   for (auto& metric : metrics_) {
     Entry::Metric* proto_metric = proto_entry->add_metrics();
     proto_metric->set_metric_hash(std::get<0>(metric));
     proto_metric->set_value(std::get<1>(metric));
   }
 }
 
 }  // namespace ukm