Verify a bit more about external strings

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 39646cd4b5e47cc2419c9fbb0a556c209dcc1440..6fb74caf97f8a84d556807c4b34d933f16e1e3e3 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -3468,14 +3468,21 @@ void ConsString::set_second(String* value, WriteBarrierMode mode) {
 }
 
 
+const void* ExternalString::resource_data() const {
+  return *reinterpret_cast<const void* const*>(
+             FIELD_ADDR_CONST(this, kResourceDataOffset));
+}
+
+
 bool ExternalString::is_short() {
   InstanceType type = map()->instance_type();
   return (type & kShortExternalStringMask) == kShortExternalStringTag;
 }
 
 
-const ExternalOneByteString::Resource* ExternalOneByteString::resource() {
-  return *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset));
+const ExternalOneByteString::Resource* ExternalOneByteString::resource() const {
+  return *reinterpret_cast<const Resource* const*>(
+             FIELD_ADDR_CONST(this, kResourceOffset));
 }
 
 
@@ -3507,8 +3514,9 @@ uint16_t ExternalOneByteString::ExternalOneByteStringGet(int index) {
 }
 
 
-const ExternalTwoByteString::Resource* ExternalTwoByteString::resource() {
-  return *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset));
+const ExternalTwoByteString::Resource* ExternalTwoByteString::resource() const {
+  return *reinterpret_cast<const Resource* const*>(
+             FIELD_ADDR_CONST(this, kResourceOffset));
 }