Cleanup: Pass std::string as const reference from ash/ + minor coding style changes in base/

ash/test/test_session_state_delegate.cc

Index: ash/test/test_session_state_delegate.cc
diff --git a/ash/test/test_session_state_delegate.cc b/ash/test/test_session_state_delegate.cc
index a2b99c85521b6e4f2c5aab47eeff236d8d2d888d..426b750232eb8272bc3dee3c3369c35fa3c1841f 100644
--- a/ash/test/test_session_state_delegate.cc
+++ b/ash/test/test_session_state_delegate.cc
@@ -99,7 +99,7 @@ TestSessionStateDelegate::~TestSessionStateDelegate() {
   STLDeleteElements(&user_list_);
 }
 
-void TestSessionStateDelegate::AddUser(const std::string user_id) {
+void TestSessionStateDelegate::AddUser(const std::string& user_id) {
   user_list_.push_back(new MockUserInfo(user_id));
 }