Subimports should block following import scripts

TEST=import-nested-dup.html, import-nested-dup-2.html
BUG=306292
R=dglazkov,esprehn

Review URL: https://codereview.chromium.org/104383002

git-svn-id: svn://svn.chromium.org/blink/trunk@163244 bbb929c8-8fbe-4397-9dbb-9b2b20218538