Currently, link preload video, audio and track resources use a single `as` value - "media" which raised a few issues when implementing the feature and in particular made it impossible to perform track-specific MIME type checks to properly support the "type" attribute with track resources.

As a result, I PRed a spec change which splits the "media" destination into 3 different destinations, as well as published a CL that implements that change.

Since it will modify shipped attribute values, this change runs a small risk of de-optimizing some content and cause some resources to not get preloaded.

We do not have use-counters for the different `as` values in place, but I've run a BigQuery query on the HTTPArchive data (with the help of Charlie, CCed), followed by a manual scan of the results and it seems only two sites in the data set have `as=media`: https://www.eatclub.com/ and http://www.io-games.org/

That seems like fairly low usage, especially since the content will not break, so I'm sending out this PSA instead of an Intent To Ship, but please let me know if an I2S is needed instead.

Thanks!

Yoav