Summary

This is a follow-up to #1951.

When testing the plugin I noticed an error in the console:

Optimization Detective] Failed to initialize extension 'http://localhost:8888/wp-content/plugins/od-intrinsic-dimensions/detect.js?ver=0.2.0': Error: Unknown element with XPath: /HTML/BODY/DIV[@class='wp-site-blocks']/*[2][self::MAIN]/*[1][self::DIV]/*[3][self::DIV]/*[1][self::FIGURE]/*[1][self::VIDEO]
    at extendElementData (detect.js?ver=1.0.0-beta4:403:9)
    at captureIntrinsicDimensions (detect.js?ver=0.2.0:34:3)
    at Module.initialize (detect.js?ver=0.2.0:72:4)
    at detect (detect.js?ver=1.0.0-beta4:821:41)

The error coming from the Intrinsic Dimensions extension started occurring after westonruter/od-intrinsic-dimensions#4 where I updated it to use initialize instead of finalize since it was deprecated in #1951 in order to facilitate gzip compression in #1959. However, I realized that the location where extensions are initialized means that they cannot call extendElementData() because the URL Metric object has not been populated yet with its elements. This causes calls to extendElementData() to always fail if they are performed while the initialize() function is being called (and not after a delay, for example).

The fix is simply to move the extension initialization logic after the URL Metric data has been initially constructed with all of the tags (elements) being tracked.