HTMLMediaElement's load() should return a Promise #554

As proposed by @cpearce in #505 (comment)

The tricky part is when the promise should be resolved. The spec has a number of steps under "Once enough of the media data has been fetched to determine the duration of the media resource, its dimensions, and other metadata" where it would make sense. At the end it says "Once the readyState attribute reaches HAVE_CURRENT_DATA, after the loadeddata event has been fired, set the element's delaying-the-load-event flag to false. This stops delaying the load event." Also resolving the load() promise at this point would probably work.