X87: Ensure object literal element boilerplates aren't modified.

port 7c347c54 (r27511)

original commit message:

    A bug allows JSObject literals with elements to have the elements in the
    boilerplate modified.

BUG=

Review URL: https://codereview.chromium.org/1057883004

Cr-Commit-Position: refs/heads/master@{#27595}