[runtime] Make elements backing store for arguments non-holey.

src/objects.cc

 // Copyright 2013 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/objects.h"
 
 #include <cmath>
 #include <iomanip>
 #include <sstream>
 
@@ skipping 14487 matching lines @@
   return used;
 }
 
 
 int JSObject::GetFastElementsUsage() {
   FixedArrayBase* store = elements();
   switch (GetElementsKind()) {
     case FAST_SMI_ELEMENTS:
     case FAST_DOUBLE_ELEMENTS:
     case FAST_ELEMENTS:
-      // Only JSArray have packed elements.
-      return Smi::cast(JSArray::cast(this)->length())->value();
+      return IsJSArray() ? Smi::cast(JSArray::cast(this)->length())->value()
+                         : store->length();
     case FAST_SLOPPY_ARGUMENTS_ELEMENTS:
       store = FixedArray::cast(FixedArray::cast(store)->get(1));
     // Fall through.
     case FAST_HOLEY_SMI_ELEMENTS:
     case FAST_HOLEY_ELEMENTS:
       return FastHoleyElementsUsage(this, FixedArray::cast(store));
     case FAST_HOLEY_DOUBLE_ELEMENTS:
       if (elements()->length() == 0) return 0;
       return FastHoleyElementsUsage(this, FixedDoubleArray::cast(store));
 
@@ skipping 3381 matching lines @@
   if (cell->value() != *new_value) {
     cell->set_value(*new_value);
     Isolate* isolate = cell->GetIsolate();
     cell->dependent_code()->DeoptimizeDependentCodeGroup(
         isolate, DependentCode::kPropertyCellChangedGroup);
   }
 }
 
 }  // namespace internal
 }  // namespace v8