Fix some missing const reference on function parameter.

cc/resources/raster_worker_pool_perftest.cc

 // Copyright 2013 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 "cc/resources/raster_worker_pool.h"
 
 #include "base/time/time.h"
 #include "cc/test/lap_timer.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/perf/perf_test.h"
@@ skipping 158 matching lines @@
               1,
               NULL,
               1,
               NULL,
               base::Bind(&RasterWorkerPoolPerfTest::OnRasterTaskCompleted),
               &decode_tasks),
           false);
     }
   }
 
-  void RunBuildTaskGraphTest(const std::string test_name,
+  void RunBuildTaskGraphTest(const std::string& test_name,
                              unsigned num_raster_tasks,
                              unsigned num_image_decode_tasks) {
     timer_.Reset();
     RasterWorkerPool::RasterTask::Queue tasks;
     CreateTasks(&tasks, num_raster_tasks, num_image_decode_tasks);
     raster_worker_pool_->SetRasterTasks(&tasks);
     do {
       raster_worker_pool_->BuildTaskGraph();
       timer_.NextLap();
     } while (!timer_.HasTimeLimitExpired());
@@ skipping 22 matching lines @@
   RunBuildTaskGraphTest("100_4", 100, 4);
   RunBuildTaskGraphTest("1000_4", 1000, 4);
   RunBuildTaskGraphTest("10_16", 10, 16);
   RunBuildTaskGraphTest("100_16", 100, 16);
   RunBuildTaskGraphTest("1000_16", 1000, 16);
 }
 
 }  // namespace
 
 }  // namespace cc