Make NodeList.length inline-cacheable by JSC.
<https://webkit.org/b/131579>

For objects with custom index or name getters, we have to make sure
that builtin properties take precedence. We do this by scanning the
ancestor chain for a suitable property slot before moving on to
named items.

With this patch, we now mark such builtins as cacheable. This is
safe since the whole point of doing this before processing named
items is to ensure the same slot is returned consistently.

Reviewed by Benjamin Poulain.

* bindings/scripts/CodeGeneratorJS.pm:
(GenerateGetOwnPropertySlotBody):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@167181 268f45cc-cd09-0410-ab3c-d52691b4dbfc