Fix some missing const reference on function parameter.

cc/resources/tile_manager_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 "base/time/time.h"
 #include "cc/resources/tile.h"
 #include "cc/resources/tile_priority.h"
 #include "cc/test/fake_output_surface.h"
 #include "cc/test/fake_output_surface_client.h"
 #include "cc/test/fake_picture_pile_impl.h"
@@ skipping 108 matching lines @@
 
   void CreateTiles(int count, TileBinVector* tiles) {
     // Roughly an equal amount of all bins.
     int count_per_bin = count / NUM_BINS;
     CreateBinTiles(count_per_bin, NOW_BIN, tiles);
     CreateBinTiles(count_per_bin, SOON_BIN, tiles);
     CreateBinTiles(count_per_bin, EVENTUALLY_BIN, tiles);
     CreateBinTiles(count - 3 * count_per_bin, NEVER_BIN, tiles);
   }
 
-  void RunManageTilesTest(const std::string test_name,
+  void RunManageTilesTest(const std::string& test_name,
                           unsigned tile_count,
                           unsigned priority_change_percent) {
     DCHECK_GE(tile_count, 100u);
     DCHECK_LE(priority_change_percent, 100u);
     TileBinVector tiles;
     CreateTiles(tile_count, &tiles);
     timer_.Reset();
     do {
       if (priority_change_percent) {
         for (unsigned i = 0;
@@ skipping 34 matching lines @@
   RunManageTilesTest("1000_10", 1000, 10);
   RunManageTilesTest("10000_10", 10000, 10);
   RunManageTilesTest("100_100", 100, 100);
   RunManageTilesTest("1000_100", 1000, 100);
   RunManageTilesTest("10000_100", 10000, 100);
 }
 
 }  // namespace
 
 }  // namespace cc