Re: I thought Chrome already did this, at least sometimes?
It does. They mentioned it. But, they make a distinction:
Passive mixed content refers to content that doesn't interact with the rest of the page, and thus a man-in-the-middle attack is restricted to what they can do if they intercept or change that content. Passive mixed content includes images, video, and audio content, along with other resources that cannot interact with the rest of the page.
I'm not sure how well they can distinguish in the browser engine though, because I know I've seen images blocked for being HTTP in HTTPS before (which I had to fix on a server).