Simplify StringCharCodeAt in non-crankshaft codegen.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index 59f9ea015f0376a57f6a5c9fc0acd84cbe40468e..880a34d35f6bc8f80147e1089dc9729cc88ce4c6 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -3647,6 +3647,9 @@ void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
 
   // Check whether the string is sequential. The only non-sequential
   // shapes we support have just been unwrapped above.
+  // Note that if the original string is a cons or slice with an external
+  // string as underlying string, we pass that unpacked underlying string with
+  // the updated index to the runtime function.
   __ bind(&check_sequential);
   STATIC_ASSERT(kSeqStringTag == 0);
   __ tst(result, Operand(kStringRepresentationMask));