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

src/arm/macro-assembler-arm.cc

Index: src/arm/macro-assembler-arm.cc
diff --git a/src/arm/macro-assembler-arm.cc b/src/arm/macro-assembler-arm.cc
index 4fc3b03abf1caced7e8009593a8c24aaeee1cdb4..bbd7c7b92070acba61000fab60a6b034129e992c 100644
--- a/src/arm/macro-assembler-arm.cc
+++ b/src/arm/macro-assembler-arm.cc
@@ -3486,8 +3486,13 @@ void MacroAssembler::EnsureNotWhite(
   ASSERT_EQ(0, kSeqStringTag & kExternalStringTag);
   ASSERT_EQ(0, kConsStringTag & kExternalStringTag);
   tst(instance_type, Operand(kExternalStringTag));
-  mov(length, Operand(ExternalString::kSize), LeaveCC, ne);
-  b(ne, &is_data_object);
+  Label not_external_string;
+  b(eq, &not_external_string);
+  tst(instance_type, Operand(kBufferedStringTag));
+  mov(length, Operand(ExternalString::kSize), LeaveCC, eq);
+  mov(length, Operand(ExternalString::kExtendedSize), LeaveCC, ne);
+  b(&is_data_object);
+  bind(&not_external_string);
 
   // Sequential string, either ASCII or UC16.
   // For ASCII (char-size of 1) we shift the smi tag away to get the length.