amp-ima-video: Support source

[aghassemi]

<amp-video-ima>
  <source src="forrest_gump.mp4" type="video/mp4">
  <source src="forrest_gump.ogg" type="video/ogg">
</amp-video-ima>

should work.

Validation rules for also need to be updated to allow amp-video-ima as parent. Rules live in validator-main.protoascii

This means data-src no longer needs to be required either.

[shawnbuso]

Taking a look at this one and I've got a quick question - do I have access to the amp-ima-video element in imaVideo.js? If I only have access to it in amp-ima-video.js, when is the earliest that I can send a message to the code in imaVideo.js to let it know what the potential sources are?

[aghassemi]

@shawnbuso adding/changing <source> later could be problematic ( see comment #9085 (comment) ) so I recommend against doing it with a postMessage.

We could scan the sources in amp-ima-video's buildCallback and serialize and put them as an attribute on the element, then when you create the 3P iframe, it would have access to that attribute same way it currently accesses the other ones.