Yesterday I could watch videos at the Khan Academy site, today they don't work. Instead of the video I get a black screen with a link to "Get Adobe Flash player". Clicking the link takes me to a page telling me "Your Google Chrome browser already includes Adobe Flash Player built-in. Google Chrome will automatically update when new versions of Flash Player are available." All sites with embedded YouTube videos have this problem.
Here's an example of a video that doesn't work: https://www.khanacademy.org/video/unit-circle-definition-of-trig-functions-1
Here's what I see instead:
And the Adobe Flash page:
The above video plays fine in Firefox, as do all other embedded YouTube videos.
Watching videos at youtube.com still works.
I have cleared everything since the beginning of time with Tools->Clear browsing data.
What has gone wrong and how can it be fixed?