Some code clean-up.

Change-Id: I4b745fd5298cd61c793e3b57514b48347bd66c0e
diff --git a/compiler/driver/compiler_driver.h b/compiler/driver/compiler_driver.h
index 0425d27..74f6835 100644
--- a/compiler/driver/compiler_driver.h
+++ b/compiler/driver/compiler_driver.h
@@ -91,7 +91,7 @@
                           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 = "");
+                          CumulativeLogger* timer, const std::string& profile_file);
 
   ~CompilerDriver();