Inherit referrer and policy when creating a nested browsing context

BUG=763194
R=estark@chromium.org

Change-Id: Ide3950269adf26ba221f573dfa088e95291ab676
Reviewed-on: https://chromium-review.googlesource.com/732652
Reviewed-by: Emily Stark <estark@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511211}
AXObjectCache* Document::ExistingAXObjectCache() const {
  auto& cache_owner = AxObjectCacheOwner();

  if (!cache_owner.GetLayoutView())
    return nullptr;

  return cache_owner.ax_object_cache_.Get();
}

AXObjectCache* Document::ExistingAXObjectCache() const {
  auto& cache_owner = AxObjectCacheOwner();

  if (!cache_owner.GetLayoutView())
    return nullptr;

  return cache_owner.ax_object_cache_.Get();
}
