Gardening: Marking registration-on-insecure-origin.html as slow

TBR=tkent
NOTRY=true
BUG=411164

Review URL: https://codereview.chromium.org/546703002

git-svn-id: svn://svn.chromium.org/blink/trunk@181438 bbb929c8-8fbe-4397-9dbb-9b2b20218538