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

net/dns/serial_worker_unittest.cc

 // Copyright (c) 2011 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.
 
 #include "net/dns/serial_worker.h"
 
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/message_loop/message_loop.h"
 #include "base/single_thread_task_runner.h"
@@ skipping 45 matching lines @@
     work_called_.Signal();
   }
 
   void OnWorkFinished() {
     EXPECT_TRUE(message_loop_ == base::MessageLoop::current());
     EXPECT_EQ(output_value_, input_value_);
     BreakNow("OnWorkFinished");
   }
 
  protected:
-  void BreakCallback(std::string breakpoint) {
+  void BreakCallback(const std::string& breakpoint) {
     breakpoint_ = breakpoint;
     base::MessageLoop::current()->QuitNow();
   }
 
-  void BreakNow(std::string b) {
+  void BreakNow(const std::string& b) {
     message_loop_->task_runner()->PostTask(
         FROM_HERE, base::Bind(&SerialWorkerTest::BreakCallback,
                               base::Unretained(this), b));
   }
 
-  void RunUntilBreak(std::string b) {
+  void RunUntilBreak(const std::string& b) {
     message_loop_->Run();
     ASSERT_EQ(breakpoint_, b);
   }
 
   SerialWorkerTest()
       : input_value_(0),
         output_value_(-1),
         work_allowed_(false, false),
         work_called_(false, false),
         work_running_(false) {
@@ skipping 68 matching lines @@
   RunUntilBreak("OnWorkFinished");
 
   // No more tasks should remain.
   EXPECT_TRUE(message_loop_->IsIdleForTesting());
 }
 
 }  // namespace
 
 }  // namespace net