Hooks for Indexed DB, StructuredSerialize

[inexorabletash]

cc: @LJWatson @siusin

• whatwg/html@915dae1 introduces a step which calls into Indexed DB, replacing imprecise spec "monkey patching" with correct behavior across the specs. Context at Specify timing of transaction deactivation more precisely IndexedDB#87

• whatwg/html@97d644c is a big change (noted as 'breaking') that replaces the StructuredClone hook with StructuredSerialize/StructuredDeserialize. Some commentary at Update for structured cloning changes in HTML IndexedDB#170

It would be good to know what the plan is for syncing with the whatwg doc is for these.

[chaals]

Needs more than a 3-minute skim to figure where this is up to in implementation. setting to milestone 7

[inexorabletash]

Update: The StructuredSerializeForStorage() hook has landed in whatwg/html and I've picked up use of it in IndexedDB.

[LJWatson]

Ping @adanilo?