HTML Imports: Make sure new imports being added to the "first child"

The notion of first child was introduced at r171966. That is,
The first import of import-tree order should take care of all
the subimports of the same URL.

This invariant is enforced by HTMLImportLoader::addImport()
but its implementation wasn't enough. This change extends it
so that the invariant enforcement is done against the subtree,
instead of just against the added node itself.

TEST=import-normalize-[123]{3}.html
R=dominicc@chromium.org,dglazkov@chromium.org
BUG=372055

Review URL: https://codereview.chromium.org/274213003

git-svn-id: svn://svn.chromium.org/blink/trunk@173900 bbb929c8-8fbe-4397-9dbb-9b2b20218538