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

ios/web/navigation/crw_session_controller_unittest.mm

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import <Foundation/Foundation.h>
 
 #include "base/logging.h"
 #import "base/mac/scoped_nsobject.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/sys_string_conversions.h"
@@ skipping 33 matching lines @@
  protected:
   void SetUp() override {
     session_controller_.reset(
         [[CRWSessionController alloc] initWithWindowName:@"test window"
                                                 openerId:@"opener"
                                              openedByDOM:NO
                                    openerNavigationIndex:0
                                             browserState:&browser_state_]);
   }
 
-  web::Referrer MakeReferrer(std::string url) {
+  web::Referrer MakeReferrer(const std::string& url) {
     return web::Referrer(GURL(url), web::ReferrerPolicyDefault);
   }
 
   web::TestWebThreadBundle thread_bundle_;
   web::TestBrowserState browser_state_;
   base::scoped_nsobject<CRWSessionController> session_controller_;
 };
 
 TEST_F(CRWSessionControllerTest, InitWithWindowName) {
   EXPECT_NSEQ(@"test window", [session_controller_ windowName]);
@@ skipping 958 matching lines @@
 
   // Remove an entry and attempt to go it. Ensure it outlives the removal.
   base::scoped_nsobject<CRWSessionEntry> entry3(
       [[session_controller_.get().entries objectAtIndex:3] retain]);
   [session_controller_ removeEntryAtIndex:3];
   [session_controller_ goToEntry:entry3];
   EXPECT_EQ(1, session_controller_.get().currentNavigationIndex);
 }
 
 }  // anonymous namespace