MediaDocumentParser should not call dispatchDocumentElementAvailable before the document element exists

When I switched MediaDocumentParser to use AttachLazily I made it call this notification
before the document element was appended. This reorders the function so it's called
at the right time. I don't think this is observable since nothing in Chromium ever
uses this hook for media documents.

Review URL: https://chromiumcodereview.appspot.com/23288006

git-svn-id: svn://svn.chromium.org/blink/trunk@156432 bbb929c8-8fbe-4397-9dbb-9b2b20218538