Cleanup: Pass std::string as const reference if possible

chrome/browser/android/history_report/delta_file_backend_leveldb.cc

Index: chrome/browser/android/history_report/delta_file_backend_leveldb.cc
diff --git a/chrome/browser/android/history_report/delta_file_backend_leveldb.cc b/chrome/browser/android/history_report/delta_file_backend_leveldb.cc
index c1903cef2df5b598bdbc1dd63bf13d9104a187bc..554d94e88ea5dab8fe447a9750c5d0b7bd6c68ac 100644
--- a/chrome/browser/android/history_report/delta_file_backend_leveldb.cc
+++ b/chrome/browser/android/history_report/delta_file_backend_leveldb.cc
@@ -37,7 +37,9 @@ int64 GetLastSeqNo(leveldb::DB* db) {
   return seq_no;
 }
 
-void SaveChange(leveldb::DB* db, std::string url, std::string type) {
+void SaveChange(leveldb::DB* db,
+                const std::string& url,
+                const std::string& type) {
   int64 seq_no = GetLastSeqNo(db) + 1;
   history_report::DeltaFileEntry entry;
   entry.set_seq_no(seq_no);