This feature will autoupgrade optionally-blockable mixed content (HTTP content in HTTPS sites) by rewriting the URL to HTTPS, without a fallback to HTTP if the content is not available over HTTPS. In the first version (targeted for M80) only audio and video content will be upgraded.
Docs: https://docs.google.com/document/d/1dp-kuN25wnEbMPNWBxM8LvOjyeydWpXPklNnGcsWK1o/edit#