Simplify overridable logic in SSLBlockingPage
Right now, SSLBlockingPage has both overridable_ and strict_enforcement_.
This CL collapses both into overridable_, so that we only need to check one.
However, strict_enforcement_ also remains on its own because it is checked
separately in one place.
BUG=
R=palmer@chromium.org
Review URL: https://codereview.chromium.org/
448163002
Cr-Commit-Position: refs/heads/master@{#288456}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288456 0039d316-1c4b-4281-b951-d872f2087c98