[Story video] Add cached sources with different qualities to amp-video

[mszylkowski]

Create sources with different bitrates for amp-video cached sources.

<amp-video>
  <source src="cache?amp_quality=high" data-bitrate="hardcoded_value">
  <source src="cache?amp_quality=medium" data-bitrate="hardcoded_value">
  <source src="cache?amp_quality=low" data-bitrate="hardcoded_value">
  <source src="origin">
</amp-video>

Pointers:

amphtml/extensions/amp-video/0.1/amp-video.js

Lines 438 to 450 in 855a2a3

	propagateCachedSources_() {
	devAssert(this.video_);
	const sources = toArray(childElementsByTag(this.element, 'source'));
	// if the `src` of `amp-video` itself is cached, move it to <source>
	if (this.element.hasAttribute('src') && this.isCachedByCDN_(this.element)) {
	const src = this.element.getAttribute('src');
	const type = this.element.getAttribute('type');
	const srcSource = this.createSourceElement_(src, type);
	const ampOrigSrc = this.element.getAttribute('amp-orig-src');
	srcSource.setAttribute('amp-orig-src', ampOrigSrc);
	// Also make sure src is removed from amp-video since Stories media-pool

amphtml/extensions/amp-video/0.1/amp-video.js

Lines 126 to 160 in 855a2a3

	/**
	* AMP Cache may selectively cache certain video sources (based on various
	* heuristics such as video type, extensions, etc...).
	* When AMP Cache does so, it rewrites the `src` for `amp-video` and
	* `source` children that are cached and adds a `amp-orig-src` attribute
	* pointing to the original source.
	*
	* There are two separate runtime concerns that we handle here:
	*
	* 1) Handling 404s
	* Eventhough AMP Cache rewrites the `src` to point to the CDN, the actual
	* video may not be ready in the cache yet, in those cases the CDN will
	* return a 404.
	* AMP Cache also rewrites Urls for all sources and returns 404 for types
	* that are not supported to be cached.
	*
	* Runtime handles this situation by appending an additional
	* <source> pointing to the original src AFTER the cached source so browser
	* will automatically proceed to the next source if one fails.
	* Original sources are added only when page becomes visible and not during
	* prerender mode.
	*
	* 2) Prerendering
	* Now that some sources might be cached, we can preload them during prerender
	* phase. Runtime handles this by adding any cached sources to the <video>
	* element during prerender and automatically sets the `preload` to `auto`
	* so browsers (based on their own heuristics) can start fetching the cached
	* videos. If `preload` is specified by the author, then it takes precedence.
	*
	* Note that this flag does not impact prerendering of the `poster` as poster
	* is fetched (and is always cached) during `buildCallback` which is not
	* dependent on the value of `prerenderAllowed()`.
	*
	* @override
	*/