Mixed Content: Start measuring the impact of blocking private IPs.

Before we know whether or not we can implement [1], let's find out how
hard it will hit the portions of the web for which we can gather UMA
data.

BUG=378566

Review URL: https://codereview.chromium.org/545923002

git-svn-id: svn://svn.chromium.org/blink/trunk@181557 bbb929c8-8fbe-4397-9dbb-9b2b20218538