XMLDocumentParser should lazy attach.

import-non-document-node.xml: whitespace text node change.
dynamic-caption-add-before-child.xhtml: this is a progression.
svg/custom/resource-invalidation-crash.svg: not clear what's going on here,
but since this is just testing for a crash with some weird patterns,
just filing a bug for now. crbug.com/284881

BUG=276689
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/23779004

git-svn-id: svn://svn.chromium.org/blink/trunk@157175 bbb929c8-8fbe-4397-9dbb-9b2b20218538