Add UseCounters for NamedNodeMap functions

This is in order to help determine which of these need to be kept in the
DOM spec, since it looks like some of them will be needed:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=21986

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/211363003

git-svn-id: svn://svn.chromium.org/blink/trunk@169969 bbb929c8-8fbe-4397-9dbb-9b2b20218538