Add webIDL dictionaries for reporting browser signals (WICG#678)

* Add webIDL dictionaries for reporting browser signals.

* Fix build error

* Apply suggestions from code review

Co-authored-by: Jeffrey Yasskin <jyasskin@gmail.com>

* Address some comments

* address comments

* Apply suggestions from code review

Co-authored-by: Jeffrey Yasskin <jyasskin@gmail.com>

* Add a todo to set interest group name for reporting

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Co-authored-by: Qingxin Wu <qingxinwu@google.com>
Co-authored-by: Jeffrey Yasskin <jyasskin@gmail.com>