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

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 5c1e9aa653684776f9a1cf4d307b941f8dbacc35..4ca3fead1782def067a3f38d354dc2791547ebff 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -14505,8 +14505,8 @@ int JSObject::GetFastElementsUsage() {
     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.