Downgrade security state while displaying an SB interstitial

BUG=615123
TBR=jialiul@chromium.org
TEST=
1. Go to the following URLs in different tabs:
   http://ianfette.org
   http://parkerly.com/sb-tests/bad-subresources.html
   https://parkerly.com/sb-tests/bad-subresources.html
2. Verify that the omnibox security indicator shows a red triangle for
   all three warnings

Review-Url: https://codereview.chromium.org/2275123004
Cr-Commit-Position: refs/heads/master@{#414700}
5 files changed