Disallow access to opaque CSS responses.

Bug: 848786
Change-Id: Ie53fbf644afdd76d7c65649a05c939c63d89b4ec
Reviewed-on: https://chromium-review.googlesource.com/1088335
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565537}
Document* CSSStyleSheet::OwnerDocument() const {
  const CSSStyleSheet* root = this;
  while (root->parentStyleSheet())
    root = root->parentStyleSheet();
  return root->ownerNode() ? &root->ownerNode()->GetDocument() : nullptr;
}

Document* CSSStyleSheet::OwnerDocument() const {
  const CSSStyleSheet* root = this;
  while (root->parentStyleSheet())
    root = root->parentStyleSheet();
  return root->ownerNode() ? &root->ownerNode()->GetDocument() : nullptr;
}
