style: Get safe area insets from Gecko.
style: Get safe area insets from Gecko.

Add binding to get safe area insets from Gecko.

Differential Revision: https://phabricator.services.mozilla.com/D52509

splav/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,splav/servo,KiChjang/servo