Add use counter for the firefox extension containsNode()

This is a firefox extension API (https://developer.mozilla.org/En/DOM/Selection).
No use counter is present for this API, so adding it here to measure the usage.

Bug=None

Review URL: https://codereview.chromium.org/518753002

git-svn-id: svn://svn.chromium.org/blink/trunk@181148 bbb929c8-8fbe-4397-9dbb-9b2b20218538