[DevTools] Show a explanation when all resources in the page have been requested securely (no-mixed-content).
* Adds a secure/green bullet point to SecurityPanel when all resources on the page have been transfered securely.
BUG=
527232
TEST=Visit 'https://sha256.badssl.com/', enable/open the Security Panel, verify there is an explanation in the list reading 'All resources on this page are served securely.' with a green bullet (secure) annotation.
Committed:
https://crrev.com/eefb629f1a7f6d05d3fede01f3dc0fb9b7f64e28
Cr-Commit-Position: refs/heads/master@{#353558}