Make arguments to insertAdjacentHTML method non-optional

http://domparsing.spec.whatwg.org/#dom-element-insertadjacenthtml

Both Firefox and IE throws when either argument is missing.

BUG=326970

Review URL: https://codereview.chromium.org/109273003

git-svn-id: svn://svn.chromium.org/blink/trunk@163519 bbb929c8-8fbe-4397-9dbb-9b2b20218538