If possible, take the constant map from the (known) native context for
JSCreateIterResultObject, so that subsequent map checks can be
eliminated in case of iterator inlining.

R=jarin@chromium.org
BUG=v8:3822

Review-Url: https://codereview.chromium.org/2394783002
Cr-Commit-Position: refs/heads/master@{#39974}