Implements the change proposed at https://github.com/tc39/ecma262/pull/593.

In summary, Object.getOwnPropertyDescriptors can produce results which cause
Object.defineProperties() to throw, by inserting a property with an undefined
descriptor into the result object. This change to the algorithm requires that
the descriptor only be added to the result object if it is not undefined.

BUG=v8:4725
R=littledan@chromium.org, adamk@chromium.org, jwolfe@igalia.com

Review-Url: https://codereview.chromium.org/2118613003
Cr-Commit-Position: refs/heads/master@{#37504}