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

chrome/browser/chromeos/settings/device_oauth2_token_service_unittest.cc

Index: chrome/browser/chromeos/settings/device_oauth2_token_service_unittest.cc
diff --git a/chrome/browser/chromeos/settings/device_oauth2_token_service_unittest.cc b/chrome/browser/chromeos/settings/device_oauth2_token_service_unittest.cc
index 66cde13725274a86efeb8c8e4445f2061f40cd1a..8460758880b8d22cf7c9cb505ed5bae42c0ea51b 100644
--- a/chrome/browser/chromeos/settings/device_oauth2_token_service_unittest.cc
+++ b/chrome/browser/chromeos/settings/device_oauth2_token_service_unittest.cc
@@ -145,7 +145,7 @@ class DeviceOAuth2TokenServiceTest : public testing::Test {
         new base::StringValue(refresh_token));
   }
 
-  std::string GetValidTokenInfoResponse(const std::string email) {
+  std::string GetValidTokenInfoResponse(const std::string& email) {
     return "{ \"email\": \"" + email + "\","
            "  \"user_id\": \"1234567890\" }";
   }