Add UKM API GetNewSourceID()

components/ukm/ukm_service.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_service.h"
 
 #include <memory>
 #include <string>
 #include <utility>
 
@@ skipping 346 matching lines @@
     return;
   }
   if (sources_.size() >= kMaxSources) {
     RecordDroppedSource(DroppedDataReason::MAX_HIT);
     return;
   }
 
   sources_.push_back(std::move(source));
 }
 
+// static
+int32_t UkmService::GetNewSourceID() {
+  static int32_t next_source_id = 0;
+  return next_source_id++;
+}
+
 std::unique_ptr<UkmEntryBuilder> UkmService::GetEntryBuilder(
     int32_t source_id,
     const char* event_name) {
   return std::unique_ptr<UkmEntryBuilder>(new UkmEntryBuilder(
       base::Bind(&UkmService::AddEntry, base::Unretained(this)), source_id,
       event_name));
 }
 
 void UkmService::UpdateSourceURL(int32_t source_id, const GURL& url) {
   DCHECK(thread_checker_.CalledOnValidThread());
@@ skipping 33 matching lines @@
   }
   if (entries_.size() >= kMaxEntries) {
     RecordDroppedEntry(DroppedDataReason::MAX_HIT);
     return;
   }
 
   entries_.push_back(std::move(entry));
 }
 
 }  // namespace ukm