AX: Share an AXObjectCache of the top document with a document in PagePopup.

This CL is a preparation to support AXObject in PagePopup.

PagePopup will set its owner element to LocalFrame::pagePopupOwner, and it
enabled to share an AXObjectCache.

- We don't set LocalFrame::pagePopupOwner yet.  This CL won't change any
  behavior.

- The goal of this CL is only to share an AXObjectCache. CLs to connect AXObject
  tree of a PagePopup to AXObject tree of the main page will follow.

BUG=123896

Review URL: https://codereview.chromium.org/501313002

git-svn-id: svn://svn.chromium.org/blink/trunk@180929 bbb929c8-8fbe-4397-9dbb-9b2b20218538