http://www.ecma-international.org/ecma-262/7.0/#sec-validateandapplypropertydescriptor
says that [[DefineProperty]] should return false if the property is
already defined and it's unconfigurable (exactly speaking, the condition
in the spec is more complicated, but roughly speaking, it's when the
property is unconfigurable).

BUG=chromium:670651

Review-Url: https://codereview.chromium.org/2680353004
Cr-Commit-Position: refs/heads/master@{#43080}