Add UseCounter for HTML Imports

BUG=383749
TEST=none
R=dglazkov@chromium.org, hayato@chromium.org

Review URL: https://codereview.chromium.org/330543007

git-svn-id: svn://svn.chromium.org/blink/trunk@176041 bbb929c8-8fbe-4397-9dbb-9b2b20218538