Use correct document for notifying of inserted import.

Notify the root document to update active stylesheets. If the import
child contains stylesheets, the StyleEngine for the import document
will be notified correctly.

Added a couple of sanity DCHECKs.

R=meade@chromium.org
BUG=671322

Review-Url: https://codereview.chromium.org/2551973003
Cr-Commit-Position: refs/heads/master@{#436887}
2 files changed