Introduce {ConstantElementsPair} struct for type safety.

src/ast/compile-time-value.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/ast/compile-time-value.h"
 
 #include "src/ast/ast.h"
 #include "src/factory.h"
 #include "src/handles-inl.h"
 #include "src/isolate.h"
@@ skipping 30 matching lines @@
   }
   return result;
 }
 
 CompileTimeValue::LiteralType CompileTimeValue::GetLiteralType(
     Handle<FixedArray> value) {
   Smi* literal_type = Smi::cast(value->get(kLiteralTypeSlot));
   return static_cast<LiteralType>(literal_type->value());
 }
 
-Handle<FixedArray> CompileTimeValue::GetElements(Handle<FixedArray> value) {
-  return Handle<FixedArray>(FixedArray::cast(value->get(kElementsSlot)));
+Handle<HeapObject> CompileTimeValue::GetElements(Handle<FixedArray> value) {
+  return Handle<HeapObject>(HeapObject::cast(value->get(kElementsSlot)));
 }
 
 }  // namespace internal
 }  // namespace v8