HTMLImportChild does not need to Document as its member.

The Document is always retrieved from the loader, which means
that we can get rid of this one, yay!

No functional changes.

R=morrita
BUG=

Review URL: https://codereview.chromium.org/294143002

git-svn-id: svn://svn.chromium.org/blink/trunk@174474 bbb929c8-8fbe-4397-9dbb-9b2b20218538