Add UseCounters for HTMLElement.innerText and .outerText

There is no specification for this yet, and there's a chance that
at least outerText is used little enough to be removed.

All major engines except Gecko have some implementation:
https://bugzilla.mozilla.org/show_bug.cgi?id=264412

BUG=none

Review URL: https://codereview.chromium.org/104713004

git-svn-id: svn://svn.chromium.org/blink/trunk@163289 bbb929c8-8fbe-4397-9dbb-9b2b20218538