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

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 1eb4640e148cf91a7ec3384ca7f0aecd19087238..b0b64fb08fb42c838ed663790fad6de2562738e5 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -110,6 +110,16 @@ inline Heap* _inline_get_heap_();
   V(Map, external_string_map, ExternalStringMap)                               \
   V(Map, external_string_with_ascii_data_map, ExternalStringWithAsciiDataMap)  \
   V(Map, external_ascii_string_map, ExternalAsciiStringMap)                    \
+  V(Map, external_buffered_symbol_map, ExternalBufferedSymbolMap)              \
+  V(Map,                                                                       \
+    external_buffered_symbol_with_ascii_data_map,                              \
+    ExternalBufferedSymbolWithAsciiDataMap)                                    \
+  V(Map, external_buffered_ascii_symbol_map, ExternalBufferedAsciiSymbolMap)   \
+  V(Map, external_buffered_string_map, ExternalBufferedStringMap)              \
+  V(Map,                                                                       \
+    external_buffered_string_with_ascii_data_map,                              \
+    ExternalBufferedStringWithAsciiDataMap)                                    \
+  V(Map, external_buffered_ascii_string_map, ExternalBufferedAsciiStringMap)   \
   V(Map, undetectable_string_map, UndetectableStringMap)                       \
   V(Map, undetectable_ascii_string_map, UndetectableAsciiStringMap)            \
   V(Map, external_pixel_array_map, ExternalPixelArrayMap)                      \