Move insertAdjacentHTML to Element

http://domparsing.spec.whatwg.org/#insertadjacenthtml()

Remove some stale comments in the process, now that we actually have a
specification. Also, IE doesn't seem to allow parsing in the void
(e.g. "afterEnd" on a detached element) anymore, at least not for
insertAdjacentHTML.

BUG=326964

Review URL: https://codereview.chromium.org/105693009

git-svn-id: svn://svn.chromium.org/blink/trunk@163529 bbb929c8-8fbe-4397-9dbb-9b2b20218538