Add a UseCounter for when xml:base takes effect in Element.baseURI

This is the only internal usage of xml:base.

BUG=341854

Review URL: https://codereview.chromium.org/697693003

git-svn-id: svn://svn.chromium.org/blink/trunk@184763 bbb929c8-8fbe-4397-9dbb-9b2b20218538