port bc3b2960 (r26172).

original commit message:
  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=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/856313002

Cr-Commit-Position: refs/heads/master@{#26179}