The local-tab logic is moving to the tab descriptor in Bug 1618693.
For now we are keeping getters on target-mixin that simply forward to the descriptor.

But we could also update the existing call sites for localTab/isLocalTab in order to directly use the descriptorFront.

The only issue is that not all targets have a descriptor (only top-level non-worker targets).
It might also be a good occasion to remove some usage for localTab.

Bug 1692874 will remove most of the targetFront::isLocalTab usage. targetFront::localTab getters are still used though.