Ensure that receiver->elements() == *dictionary after calling an accessor, in
addition to checking the prototype.

BUG=chromium:634273, chromium: 634357, v8:5162
R=cbruni@chromium.org, mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2212963002
Cr-Commit-Position: refs/heads/master@{#38347}