After [1] we return JSArray with internal structs, we should return JSObjects instead.

[1] https://codereview.chromium.org/2789073002

BUG=v8:6189
R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2806373005
Cr-Commit-Position: refs/heads/master@{#44581}