Pass some arguments by const reference.

Test: Rely on TreeHugger.
Bug: 32619234
Change-Id: Ia26277893641dcb3562eaeacaec4e089a978d56a
diff --git a/runtime/runtime.h b/runtime/runtime.h
index 77ec238..6f68960 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -182,7 +182,7 @@
     return compiler_options_;
   }
 
-  void AddCompilerOption(std::string option) {
+  void AddCompilerOption(const std::string& option) {
     compiler_options_.push_back(option);
   }