Getter properties are not currently included in the protocol's
Runtime.ObjectPreview. DevTools currently shows getter properties
when evaluating arrays in the console, and this CL brings them into
the preview generated for RemoteObjects.

Corresponding DevTools CL: https://codereview.chromium.org/2521513006/

BUG=666882

Review-Url: https://codereview.chromium.org/2508423002
Cr-Commit-Position: refs/heads/master@{#41565}