Fix issue with __proto__ when using ES6 object literals

Fix issue with __proto__ when using ES6 object literals

It should be possible to create a concise method with the name
__proto__ without setting the [[Prototype]]. Similarly, property
name shorthands with the name __proto__ should define an own
property.

BUG=v8:3818
LOG=Y
R=adamk, dslomov@chromium.org

Review URL: https://codereview.chromium.org/858673002

Cr-Commit-Position: refs/heads/master@{#26172}