[import] Add a null check in HTMLImportChild::ownsLoader()

This is a speculative crash fix.

TEST=none
BUG=342953
R=dominicc@chromium.org

Review URL: https://codereview.chromium.org/160093005

git-svn-id: svn://svn.chromium.org/blink/trunk@166992 bbb929c8-8fbe-4397-9dbb-9b2b20218538