Throw DOM exception in insertAdjacentHTML if context becomes null

Align first part of the insertAdjacentHTML implementation with
http://domparsing.spec.whatwg.org/#insertadjacenthtml().

"Let context be the context object's parent.

 If context is null or a document, throw a DOMException with name
 NoModificationAllowedError and terminate these steps."

While the 'or a document' part was previously implemented, the
'context is null' part was not.

BUG=326965

Review URL: https://codereview.chromium.org/110383002

git-svn-id: svn://svn.chromium.org/blink/trunk@163452 bbb929c8-8fbe-4397-9dbb-9b2b20218538