Remove HTMLHtmlElement.manifest

Intent to Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/5nQVw4T1EJw/FURw6bIpwvoJ

BUG=none

Review URL: https://codereview.chromium.org/282073008

git-svn-id: svn://svn.chromium.org/blink/trunk@174566 bbb929c8-8fbe-4397-9dbb-9b2b20218538