DedicatedWorkerGlobalScope is an object with a Global extended
attribute, but does not define any named property getters. This needs to
be handled by setting the prototype chain to:
DedicatedWorkerGlobalScope
^ DedicatedWorkerGlobalScopePrototype
^ WorkerGlobalScopePrototype
(This is different from something like Window, where there is an
intermediate WindowProperties object for named properties.)
Previously, we treated the GlobalMixin object as if it was a simple
prototype object, accidentally setting DedicatedWorkerGlobalScope's
prototype to WorkerGlobalScopePrototype. This caused the expression
self instanceof DedicatedWorkerGlobalScope
to return false inside workers.
This makes us pass many more of the "/xhr/idlharness.any.worker" WPT
tests than before, rather than failing early.
