HTML API: Add method to report depth of currently-matched element. #5428

Job	Run time
Core running from build / Core running from build / Linux	1m 54s
Core running from src / Core running from src / Linux	1m 48s
Core running from build / Core running from build / Windows	3m 44s
Core running from src / Core running from src / Windows	3m 46s
Core running from build / Core running from build / MacOS	1m 39s
Core running from src / Core running from src / MacOS	2m 27s
Gutenberg running from build / Gutenberg running from build / Linux	8m 43s
Gutenberg running from src / Gutenberg running from src / Linux	8m 37s
Gutenberg running from build / Gutenberg running from build / Windows	12m 1s
Gutenberg running from src / Gutenberg running from src / Windows	11m 36s
Gutenberg running from build / Gutenberg running from build / MacOS	9m 44s
Gutenberg running from src / Gutenberg running from src / MacOS	7m 57s
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	1h 13m 58s

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