This makes Object.getOwnPropertyNames() return the integer keys in the
proper order, following the spec:

http://www.ecma-international.org/ecma-262/6.0/#sec-ordinary-object-internal-methods-and-internal-slots-ownpropertykeys

BUG=v8:4118
LOG=n

Review URL: https://codereview.chromium.org/1228803006

Cr-Commit-Position: refs/heads/master@{#29667}