Some code clean-up.

Change-Id: I4b745fd5298cd61c793e3b57514b48347bd66c0e
diff --git a/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index aac33d2..4b1063f 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -332,7 +332,7 @@
                                const InstructionSetFeatures* instruction_set_features,
                                bool image, std::set<std::string>* image_classes, size_t thread_count,
                                bool dump_stats, bool dump_passes, CumulativeLogger* timer,
-                               std::string profile_file)
+                               const std::string& profile_file)
     : profile_present_(false), compiler_options_(compiler_options),
       verification_results_(verification_results),
       method_inliner_map_(method_inliner_map),