Section 8.10.5 9a specifies that a property descriptor cannot both have
accessors and specify the writability of the property. The previous
error message was misleading because it referred to writable rather
than specifying the writability (which includes writable: false).

BUG=v8:2536
LOG=N

Review URL: https://codereview.chromium.org/1399693003

Cr-Commit-Position: refs/heads/master@{#31273}