Add a UseCounter for the overflowchanged event

By request from PhistucK:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/uaQHXAtaQCg/m46ZdQU3K1IJ

BUG=none

Review URL: https://codereview.chromium.org/102793003

git-svn-id: svn://svn.chromium.org/blink/trunk@163128 bbb929c8-8fbe-4397-9dbb-9b2b20218538