Fixed bug where PlzNavigate CSP in a iframe did not get the inherited CSP

When inheriting the CSP from a parent document to a local-scheme CSP,
it does not always get propagated to the PlzNavigate CSP. This means
that PlzNavigate CSP checks (like `frame-src`) would be ran against
a blank policy instead of the proper inherited policy.

Bug: 778658
Change-Id: I61bb0d432e1cea52f199e855624cb7b3078f56a9
Reviewed-on: https://chromium-review.googlesource.com/765969
Commit-Queue: Andy Paicu <andypaicu@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518245}
Element* Document::ElementFromPoint(double x, double y) const {
  if (GetLayoutViewItem().IsNull())
    return nullptr;

  return TreeScope::ElementFromPoint(x, y);
}

Element* Document::ElementFromPoint(double x, double y) const {
  if (GetLayoutViewItem().IsNull())
    return nullptr;

  return TreeScope::ElementFromPoint(x, y);
}
