Introduce read buffer for external strings when using charAt (ia32).

src/code-stubs.h

Index: src/code-stubs.h
diff --git a/src/code-stubs.h b/src/code-stubs.h
index 83b0f49140e07de902782721d0a30be6cf337789..5276ce70843626f74ac180baa2a0435db6d48a23 100644
--- a/src/code-stubs.h
+++ b/src/code-stubs.h
@@ -794,6 +794,15 @@ class StringCharCodeAtGenerator {
   void GenerateSlow(MacroAssembler* masm,
                     const RuntimeCallHelper& call_helper);
 
+  // Generates the code for handling different string types and loading the
+  // result character.  We expect |index| to be not smi-tagged.
+  static void GenerateCharLoad(MacroAssembler* masm,
+                               Factory* factory,
+                               Register string,
+                               Register index,
+                               Register result,
+                               Label* call_runtime);
+
  private:
   Register object_;
   Register index_;