Add HTMLImportChild::link()

This API will be used to DevTool import support.

R=dglazkov@chromium.org,esprehn@chromium.org
BUG=340319
TEST=none

Review URL: https://codereview.chromium.org/157033004

git-svn-id: svn://svn.chromium.org/blink/trunk@166664 bbb929c8-8fbe-4397-9dbb-9b2b20218538