Add UKM API GetNewSourceID()

components/ukm/ukm_service.cc

Index: components/ukm/ukm_service.cc
diff --git a/components/ukm/ukm_service.cc b/components/ukm/ukm_service.cc
index 06c3dbb5f59a60870f12b910eca9dd3289dc12db..80e1c389706ae65fe3b5b299ed8a76a24ccf1a20 100644
--- a/components/ukm/ukm_service.cc
+++ b/components/ukm/ukm_service.cc
@@ -364,6 +364,12 @@ void UkmService::RecordSource(std::unique_ptr<UkmSource> source) {
   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) {