rMOZILLARELEASE72c9cdfb1049

Bug 1646547 - fix ScriptLoader::ReadyToExecuteParserBlockingScripts stopping at OOP ancestors. r=kmag

`ReadyToExecuteParserBlockingScripts` was walking the ancestor chain but
would stop if it hit an OOP ancestor. With this change we walk the
`WindowContext` tree instead, so that we may skip over OOP ancestors
and continue checking all in process ancestors.

Differential Revision: https://phabricator.services.mozilla.com/D86436