Avoid some unnecessary use of stl types.

May be a minor performance win but really done for libc++ as this removes a
dependency on a library function whose prototype includes a std::string.

Change-Id: Ifc07a9caef1206caf2a69a6e1b0bfc0ba56c8c62
diff --git a/runtime/well_known_classes.h b/runtime/well_known_classes.h
index bc928d0..f6c2930 100644
--- a/runtime/well_known_classes.h
+++ b/runtime/well_known_classes.h
@@ -54,6 +54,7 @@
   static jclass java_lang_reflect_Proxy;
   static jclass java_lang_RuntimeException;
   static jclass java_lang_StackOverflowError;
+  static jclass java_lang_String;
   static jclass java_lang_System;
   static jclass java_lang_Thread;
   static jclass java_lang_ThreadGroup;