Introduce {ConstantElementsPair} struct for type safety.

src/full-codegen/arm/full-codegen-arm.cc

Index: src/full-codegen/arm/full-codegen-arm.cc
diff --git a/src/full-codegen/arm/full-codegen-arm.cc b/src/full-codegen/arm/full-codegen-arm.cc
index 2408fe2cc27f8437edc7bc2a3930090a9c651764..10adc7e9054890b387b8e802c6a5abbdf2debfc1 100644
--- a/src/full-codegen/arm/full-codegen-arm.cc
+++ b/src/full-codegen/arm/full-codegen-arm.cc
@@ -1333,11 +1333,9 @@ void FullCodeGenerator::VisitObjectLiteral(ObjectLiteral* expr) {
 void FullCodeGenerator::VisitArrayLiteral(ArrayLiteral* expr) {
   Comment cmnt(masm_, "[ ArrayLiteral");
 
-  Handle<FixedArray> constant_elements = expr->constant_elements();
+  Handle<ConstantElementsPair> constant_elements = expr->constant_elements();
   bool has_fast_elements =
       IsFastObjectElementsKind(expr->constant_elements_kind());
-  Handle<FixedArrayBase> constant_elements_values(
-      FixedArrayBase::cast(constant_elements->get(1)));
 
   AllocationSiteMode allocation_site_mode = TRACK_ALLOCATION_SITE;
   if (has_fast_elements && !FLAG_allocation_site_pretenuring) {