Refactoring: Get rid of HTMLImportLoaerClient

This indirection doesn't help and just introduces unnecessary complexity.

BUG=356257
TEST=none
R=dominicc@chromium.org,dglazkov@chromium.org

Review URL: https://codereview.chromium.org/211663002

git-svn-id: svn://svn.chromium.org/blink/trunk@170006 bbb929c8-8fbe-4397-9dbb-9b2b20218538