Remove Node::ancestorElement.

Ancestor element is a DOM concept that is not in DOM4. (DOM4 has
"ancestors", but it's a set of elements.) Every callsite asked for the
ancestor element of an element, which is effectively the same as
parentElement in Blink. So use parentElement instead.

BUG=325922

Review URL: https://codereview.chromium.org/130013002

git-svn-id: svn://svn.chromium.org/blink/trunk@164824 bbb929c8-8fbe-4397-9dbb-9b2b20218538