Remove indirection plumbing from HTMLImportChild.

The indirection plumbing obscures what is really happening at the callsites
and should be removed. Also, strengthen up the assertions, since
now HTMLImportChild has HTMLImportLoader for (observable) life.

R=morrita@chromium.org
BUG=

Review URL: https://codereview.chromium.org/299313013

git-svn-id: svn://svn.chromium.org/blink/trunk@175244 bbb929c8-8fbe-4397-9dbb-9b2b20218538