Minimal AMP support: don't invalidate AMP #1263
Description
PageSpeed should never inject JavaScript into AMP pages, as that will invalidate them. I think to determine that we need to look for <html AMP>, although the spec which has a utf-8 lightning bolt character rather than "AMP": <html ⚡>. See https://www.ampproject.org/docs/get_started/about-amp.html . (how does that work when the site is in russian or chinese encoding???)
PageSpeed should optimize src in amp-img tags, which starts with adding some default UrlValuedAttribute settings. See https://www.ampproject.org/docs/reference/amp-img.html . But for the moment, PageSpeed should not transcode amp-img URLs to webp because AMP pages must be served as publicly cacheable without varying on user-agent. So some kind of