Measure the usage of prefixed Shadow DOM APIs.

BUG=242153
R=hayato@chromium.org

Review URL: https://chromiumcodereview.appspot.com/20249002

git-svn-id: svn://svn.chromium.org/blink/trunk@154901 bbb929c8-8fbe-4397-9dbb-9b2b20218538