Remove redundancy in security interstitial UMA logic
Currently, very similar events are being recorded in different ways between the
Safe Browsing and SSL interstitials. This unifies the event types and shares a
common helper method between the two. It should be extensible to future
interstitials. It also has the pleasant side effect of reducing the overwhelming
complexity of the previous interstitial.ssl histogram.
BUG=448487
Review URL: https://codereview.chromium.org/
839183002
Cr-Commit-Position: refs/heads/master@{#312042}