Added usecounters for the various picture element parts.

At the request of Paul Irish, I've added use counters to the various
picture element feature parts, so that their usage can be displayed on
chromestatus.com.

BUG=408797

Review URL: https://codereview.chromium.org/526503003

git-svn-id: svn://svn.chromium.org/blink/trunk@181131 bbb929c8-8fbe-4397-9dbb-9b2b20218538