Setting loading=eager should be less COMEFROM-ey

[domfarolino]

Currently a loading=lazy image is deferred until the will lazy load image steps return false, and these steps return true as long as an image's loading attribute is in the Lazy state. This means right when an element's loading attribute is set to the Eager state, these steps will return false.

While that's not too COMEFROM-ey (since we're explicitly invoking the steps that apparently observe the state of the loading attribute), I think it would be clearer if we explicitly resumed the #updating-the-image-data algorithm when the loading attribute gets set to the Eager state.

This is filed as a follow-up to #5510 (comment), and I'll take care of it when that PR lands.

[domfarolino]

Closed by by #5573