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

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 64bda9473f6ca4d98232046094c5faf560d0f97e..c3773369e9cdc90571c8325edb997d54891ef4bc 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -380,6 +380,8 @@ void String::StringVerify() {
     ConsString::cast(this)->ConsStringVerify();
   } else if (IsSlicedString()) {
     SlicedString::cast(this)->SlicedStringVerify();
+  } else if (IsExternalString()) {
+    ExternalString::cast(this)->ExternalStringVerify();
   }
 }
 
@@ -404,6 +406,15 @@ void SlicedString::SlicedStringVerify() {
 }
 
 
+void ExternalString::ExternalStringVerify() {
+  if (IsBuffered()) {
+    CHECK(length() >= kMinBufferedStringLength);
+    int index = buffer_index();
+    CHECK(index == kInvalidBufferIndex || (index >= 0 && index < length()));
+  }
+}
+
+
 void JSFunction::JSFunctionVerify() {
   CHECK(IsJSFunction());
   VerifyObjectField(kPrototypeOrInitialMapOffset);