Measure ad density based on document height, not viewport area

[warrengm]

Reduce ad density in initial viewport audit measures ad density in the initial viewport based on area while industry best practices usually discuss density vertically based on content length. Do note that "content length" is somewhat vague so it's likely hard to define precisely and will need some further discussion

See https://www.betterads.org/mobile-ad-density-higher-than-30/ and https://developers.google.com/publisher-ads-audits/reference/audits/viewport-ad-density#fn1