Fix some missing const reference on function parameter.

cc/resources/worker_pool_perftest.cc

Index: cc/resources/worker_pool_perftest.cc
diff --git a/cc/resources/worker_pool_perftest.cc b/cc/resources/worker_pool_perftest.cc
index 346b291df5228d4f3ca6c70d7d6b88b8f55657e6..06a2b9806df4825275078301a3701305b120e6bc 100644
--- a/cc/resources/worker_pool_perftest.cc
+++ b/cc/resources/worker_pool_perftest.cc
@@ -160,13 +160,13 @@ class WorkerPoolPerfTest : public testing::Test {
     worker_pool_->CheckForCompletedTasks();
   }
 
-  void AfterTest(const std::string test_name) {
+  void AfterTest(const std::string& test_name) {
     // Format matches chrome/test/perf/perf_test.h:PrintResult
     printf(
         "*RESULT %s: %.2f runs/s\n", test_name.c_str(), timer_.LapsPerSecond());
   }
 
-  void RunScheduleTasksTest(const std::string test_name,
+  void RunScheduleTasksTest(const std::string& test_name,
                             unsigned max_depth,
                             unsigned num_children_per_node) {
     timer_.Reset();
@@ -186,7 +186,7 @@ class WorkerPoolPerfTest : public testing::Test {
                            timer_.LapsPerSecond(), "runs/s", true);
   }
 
-  void RunExecuteTasksTest(const std::string test_name,
+  void RunExecuteTasksTest(const std::string& test_name,
                            unsigned max_depth,
                            unsigned num_children_per_node) {
     timer_.Reset();