AX: Add HTMLInputElement::popupRootAXObject, and make a path to get a PagePopup AXObject.

This CL is a preparation to support AXObject in PagePopup.
Adds the followings to pass a PagePopup AXObject to the main document.
 - HTMLInputElement::popupRootAXObject
 - InputTypeView::popupRootAXObject
 - PickerIndicatorElement::popupRootAXObject
 - DateTimeChooser::rootAXObject
 - ColorChooser::rootAXObject
 - PagePopup::rootAXObject

HTMLInputElement::popupRootAXObject will be called from
AXRenderObject::addChildren.

BUG=123896

Review URL: https://codereview.chromium.org/509833002

git-svn-id: svn://svn.chromium.org/blink/trunk@180939 bbb929c8-8fbe-4397-9dbb-9b2b20218538