Add a UseCounter for HTMLHtmlElement.manifest

http://whatwg.org/html#attr-html-manifest

The spec says "The manifest attribute only has an effect during the
early stages of document load. Changing the attribute dynamically thus
has no effect (and thus, no DOM API is provided for this attribute)."

Added in <http://trac.webkit.org/changeset/66327> and confirmed by
Darin Adler to be unintentional.

BUG=325420

Review URL: https://codereview.chromium.org/100953004

git-svn-id: svn://svn.chromium.org/blink/trunk@163170 bbb929c8-8fbe-4397-9dbb-9b2b20218538