Add a UseCounter for the webkitallowfullscreen attribute

We support the unprefixed allowfullscreen attribute, so it would be nice
to also reflect it in HTMLIFrameElement.allowFullscreen. However, that's
tricky while there are two backing attributes, so first measure if the
prefixed attribute can be removed yet.

BUG=none

Review URL: https://codereview.chromium.org/212433004

git-svn-id: svn://svn.chromium.org/blink/trunk@170124 bbb929c8-8fbe-4397-9dbb-9b2b20218538