This changes PipelineId to be a UUID and removes the concept of SubpageId.

Iframes are now able to generate their own pipeline ID during navigation which simplifies a lot of logic in other parts of the code.

Other parts of the code, such as the compositor and layout only ever deal with pipeline ids.

…eate pipeline IDs.

This is currently the constellation, and each script thread that is created.

This guarantees that pipeline IDs are unique, without relying on UUIDs.

Each time a new script thread is created, the constellation passes through a new namespace identifier.