Use "exists" rather than "is present" in dictionary checks.

[rakuco]

whatwg/webidl#859 removed the "present" and "not present" terms; specs are supposed to use Infra's "exists" for dictionaries instead.

We had already done this in #440, but there were some leftovers in the Automation section.

Related to #444.

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[kenchris]

Lgtm