cleanup: pass string as const reference from c/b/extension/api

chrome/browser/extensions/api/spellcheck/spellcheck_api.cc

Index: chrome/browser/extensions/api/spellcheck/spellcheck_api.cc
diff --git a/chrome/browser/extensions/api/spellcheck/spellcheck_api.cc b/chrome/browser/extensions/api/spellcheck/spellcheck_api.cc
index 3f88362d775a817b97e54ddc712b2281f4d112d1..56e4d658614f16da98b049b4e114a2221ad83f7e 100644
--- a/chrome/browser/extensions/api/spellcheck/spellcheck_api.cc
+++ b/chrome/browser/extensions/api/spellcheck/spellcheck_api.cc
@@ -25,7 +25,8 @@ SpellcheckDictionaryInfo* GetSpellcheckDictionaryInfo(
   return spellcheck_info;
 }
 
-SpellcheckService::DictionaryFormat GetDictionaryFormat(std::string format) {
+SpellcheckService::DictionaryFormat GetDictionaryFormat(
+    const std::string& format) {
   if (format == "hunspell") {
     return SpellcheckService::DICT_HUNSPELL;
   } else if (format == "text") {