Use generated code to create sub strings

src/runtime.cc

Index: src/runtime.cc
===================================================================
--- src/runtime.cc	(revision 3551)
+++ src/runtime.cc	(working copy)
@@ -2473,6 +2473,7 @@
   RUNTIME_ASSERT(end >= start);
   RUNTIME_ASSERT(start >= 0);
   RUNTIME_ASSERT(end <= value->length());
+  Counters::sub_string_runtime.Increment();
   return value->SubString(start, end);
 }